CHEMISTRY SINCE THE TIME OF DALTON
JOHN DALTON AND THE ATOMIC THEORY
Small beginnings as have great endings--sometimes. As a case in
point, note what came of the small, original effort of a
self-trained back-country Quaker youth named John Dalton, who
along towards the close of the eighteenth century became
interested in the weather, and was led to construct and use a
crude water-gauge to test the amount of the rainfall. The simple
experiments thus inaugurated led to no fewer than two hundred
thousand recorded observations regarding the weather, which
formed the basis for some of the most epochal discoveries in
meteorology, as we have seen. But this was only a beginning. The
simple rain-gauge pointed the way to the most important
generalization of the nineteenth century in a field of science
with which, to the casual observer, it might seem to have no
alliance whatever. The wonderful theory of atoms, on which the
whole gigantic structure of modern chemistry is founded, was the
logical outgrowth, in the mind of John Dalton, of those early
studies in meteorology.
The way it happened was this: From studying the rainfall, Dalton
turned naturally to the complementary process of evaporation. He
was soon led to believe that vapor exists, in the atmosphere as
an independent gas. But since two bodies cannot occupy the same
space at the same time, this implies that the various atmospheric
gases are really composed of discrete particles. These ultimate
particles are so small that we cannot see them--cannot, indeed,
more than vaguely imagine them--yet each particle of vapor, for
example, is just as much a portion of water as if it were a drop
out of the ocean, or, for that matter, the ocean itself. But,
again, water is a compound substance, for it may be separated, as
Cavendish has shown, into the two elementary substances hydrogen
and oxygen. Hence the atom of water must be composed of two
lesser atoms joined together. Imagine an atom of hydrogen and one
of oxygen. Unite them, and we have an atom of water; sever them,
and the water no longer exists; but whether united or separate
the atoms of hydrogen and of oxygen remain hydrogen and oxygen
and nothing else. Differently mixed together or united, atoms
produce different gross substances; but the elementary atoms
never change their chemical nature--their distinct personality.
It was about the year 1803 that Dalton first gained a full grasp
of the conception of the chemical atom. At once he saw that the
hypothesis, if true, furnished a marvellous key to secrets of
matter hitherto insoluble--questions relating to the relative
proportions of the atoms themselves. It is known, for example,
that a certain bulk of hydrogen gas unites with a certain bulk of
oxygen gas to form water. If it be true that this combination
consists essentially of the union of atoms one with another (each
single atom of hydrogen united to a single atom of oxygen), then
the relative weights of the original masses of hydrogen and of
oxygen must be also the relative weights of each of their
respective atoms. If one pound of hydrogen unites with five and
one-half pounds of oxygen (as, according to Dalton's experiments,
it did), then the weight of the oxygen atom must be five and
one-half times that of the hydrogen atom. Other compounds may
plainly be tested in the same way. Dalton made numerous tests
before he published his theory. He found that hydrogen enters
into compounds in smaller proportions than any other element
known to him, and so, for convenience, determined to take the
weight of the hydrogen atom as unity. The atomic weight of
oxygen then becomes (as given in Dalton's first table of 1803)
5.5; that of water (hydrogen plus oxygen) being of course 6.5.
The atomic weights of about a score of substances are given in
Dalton's first paper, which was read before the Literary and
Philosophical Society of Manchester, October 21, 1803. I wonder
if Dalton himself, great and acute intellect though he had,
suspected, when he read that paper, that he was inaugurating one
of the most fertile movements ever entered on in the whole
history of science?
Be that as it may, it is certain enough that Dalton's
contemporaries were at first little impressed with the novel
atomic theory. Just at this time, as it chanced, a dispute was
waging in the field of chemistry regarding a matter of empirical
fact which must necessarily be settled before such a theory as
that of Dalton could even hope for a bearing. This was the
question whether or not chemical elements unite with one another
always in definite proportions. Berthollet, the great co-worker
with Lavoisier, and now the most authoritative of living
chemists, contended that substances combine in almost
indefinitely graded proportions between fixed extremes. He held
that solution is really a form of chemical combination--a
position which, if accepted, left no room for argument.
But this contention of the master was most actively disputed, in
particular by Louis Joseph Proust, and all chemists of repute
were obliged to take sides with one or the other. For a time the
authority of Berthollet held out against the facts, but at last
accumulated evidence told for Proust and his followers, and
towards the close of the first decade of our century it came to
be generally conceded that chemical elements combine with one
another in fixed and definite proportions.
More than that. As the analysts were led to weigh carefully the
quantities of combining elements, it was observed that the
proportions are not only definite, but that they bear a very
curious relation to one another. If element A combines with two
different proportions of element B to form two compounds, it
appears that the weight of the larger quantity of B is an exact
multiple of that of the smaller quantity. This curious relation
was noticed by Dr. Wollaston, one of the most accurate of
observers, and a little later it was confirmed by Johan Jakob
Berzelius, the great Swedish chemist, who was to be a dominating
influence in the chemical world for a generation to come. But
this combination of elements in numerical proportions was exactly
what Dalton had noticed as early as 1802, and what bad led him
directly to the atomic weights. So the confirmation of this
essential point by chemists of such authority gave the strongest
confirmation to the atomic theory.
During these same years the rising authority of the French
chemical world, Joseph Louis Gay-Lussac, was conducting
experiments with gases, which he had undertaken at first in
conjunction with Humboldt, but which later on were conducted
independently. In 1809, the next year after the publication of
the first volume of Dalton's New System of Chemical Philosophy,
Gay-Lussac published the results of his observations, and among
other things brought out the remarkable fact that gases, under
the same conditions as to temperature and pressure, combine
always in definite numerical proportions as to volume. Exactly
two volumes of hydrogen, for example, combine with one volume of
oxygen to form water. Moreover, the resulting compound gas
always bears a simple relation to the combining volumes. In the
case just cited, the union of two volumes of hydrogen and one of
oxygen results in precisely two volumes of water vapor.
Naturally enough, the champions of the atomic theory seized upon
these observations of Gay-Lussac as lending strong support to
their hypothesis--all of them, that is, but the curiously
self-reliant and self-sufficient author of the atomic theory
himself, who declined to accept the observations of the French
chemist as valid. Yet the observations of Gay-Lussac were
correct, as countless chemists since then have demonstrated anew,
and his theory of combination by volumes became one of the
foundation-stones of the atomic theory, despite the opposition of
the author of that theory.
The true explanation of Gay-Lussac's law of combination by
volumes was thought out almost immediately by an Italian savant,
Amadeo, Avogadro, and expressed in terms of the atomic theory.
The fact must be, said Avogadro, that under similar physical
conditions every form of gas contains exactly the same number of
ultimate particles in a given volume. Each of these ultimate
physical particles may be composed of two or more atoms (as in
the case of water vapor), but such a compound atom conducts
itself as if it were a simple and indivisible atom, as regards
the amount of space that separates it from its fellows under
given conditions of pressure and temperature. The compound atom,
composed of two or more elementary atoms, Avogadro proposed to
distinguish, for purposes of convenience, by the name molecule.
It is to the molecule, considered as the unit of physical
structure, that Avogadro's law applies.
This vastly important distinction between atoms and molecules,
implied in the law just expressed, was published in 1811. Four
years later, the famous French physicist Ampere outlined a
similar theory, and utilized the law in his mathematical
calculations. And with that the law of Avogadro dropped out of
sight for a full generation. Little suspecting that it was the
very key to the inner mysteries of the atoms for which they were
seeking, the chemists of the time cast it aside, and let it fade
from the memory of their science.
This, however, was not strange, for of course the law of Avogadro
is based on the atomic theory, and in 1811 the atomic theory was
itself still being weighed in the balance. The law of multiple
proportions found general acceptance as an empirical fact; but
many of the leading lights of chemistry still looked askance at
Dalton's explanation of this law. Thus Wollaston, though from the
first he inclined to acceptance of the Daltonian view, cautiously
suggested that it would be well to use the non-committal word
"equivalent" instead of "atom"; and Davy, for a similar reason,
in his book of 1812, speaks only of "proportions," binding
himself to no theory as to what might be the nature of these
proportions.
At least two great chemists of the time, however, adopted the
atomic view with less reservation. One of these was Thomas
Thomson, professor at Edinburgh, who, in 1807, had given an
outline of Dalton's theory in a widely circulated book, which
first brought the theory to the general attention of the chemical
world. The other and even more noted advocate of the atomic
theory was Johan Jakob Berzelius. This great Swedish chemist at
once set to work to put the atomic theory to such tests as might
be applied in the laboratory. He was an analyst of the utmost
skill, and for years be devoted himself to the determination of
the combining weights, "equivalents" or "proportions," of the
different elements. These determinations, in so far as they were
accurately made, were simple expressions of empirical facts,
independent of any theory; but gradually it became more and more
plain that these facts all harmonize with the atomic theory of
Dalton. So by common consent the proportionate combining weights
of the elements came to be known as atomic weights--the name
Dalton had given them from the first--and the tangible conception
of the chemical atom as a body of definite constitution and
weight gained steadily in favor.
From the outset the idea had had the utmost tangibility in the
mind of Dalton. He had all along represented the different atoms
by geometrical symbols--as a circle for oxygen, a circle
enclosing a dot for hydrogen, and the like--and had represented
compounds by placing these symbols of the elements in
juxtaposition. Berzelius proposed to improve upon this method by
substituting for the geometrical symbol the initial of the Latin
name of the element represented--O for oxygen, H for hydrogen,
and so on--a numerical coefficient to follow the letter as an
indication of the number of atoms present in any given compound.
This simple system soon gained general acceptance, and with
slight modifications it is still universally employed. Every
school-boy now is aware that H2O is the chemical way of
expressing the union of two atoms of hydrogen with one of oxygen
to form a molecule of water. But such a formula would have had
no meaning for the wisest chemist before the day of Berzelius.
The universal fame of the great Swedish authority served to give
general currency to his symbols and atomic weights, and the new
point of view thus developed led presently to two important
discoveries which removed the last lingering doubts as to the
validity of the atomic theory. In 1819 two French physicists,
Dulong and Petit, while experimenting with heat, discovered that
the specific heats of solids (that is to say, the amount of heat
required to raise the temperature of a given mass to a given
degree) vary inversely as their atomic weights. In the same year
Eilhard Mitscherlich, a German investigator, observed that
compounds having the same number of atoms to the molecule are
disposed to form the same angles of crystallization--a property
which he called isomorphism.
Here, then, were two utterly novel and independent sets of
empirical facts which harmonize strangely with the supposition
that substances are composed of chemical atoms of a determinate
weight. This surely could not be coincidence--it tells of law.
And so as soon as the claims of Dulong and Petit and of
Mitscherlich had been substantiated by other observers, the laws
of the specific heat of atoms, and of isomorphism, took their
place as new levers of chemical science. With the aid of these
new tools an impregnable breastwork of facts was soon piled about
the atomic theory. And John Dalton, the author of that theory,
plain, provincial Quaker, working on to the end in
semi-retirement, became known to all the world and for all time
as a master of masters.
HUMPHRY DAVY AND ELECTRO-CHEMISTRY
During those early years of the nineteenth century, when Dalton
was grinding away at chemical fact and theory in his obscure
Manchester laboratory, another Englishman held the attention of
the chemical world with a series of the most brilliant and widely
heralded researches. This was Humphry Davy, a young man who had
conic to London in 1801, at the instance of Count Rumford, to
assume the chair of chemical philosophy in the Royal Institution,
which the famous American had just founded.
Here, under Davy's direction, the largest voltaic battery yet
constructed had been put in operation, and with its aid the
brilliant young experimenter was expected almost to perform
miracles. And indeed he scarcely disappointed the expectation,
for with the aid of his battery he transformed so familiar a
substance as common potash into a metal which was not only so
light that it floated on water, but possessed the seemingly
miraculous property of bursting into flames as soon as it came in
contact with that fire-quenching liquid. If this were not a
miracle, it had for the popular eye all the appearance of the
miraculous.
What Davy really had done was to decompose the potash, which
hitherto had been supposed to be elementary, liberating its
oxygen, and thus isolating its metallic base, which he named
potassium. The same thing was done with soda, and the closely
similar metal sodium was discovered--metals of a unique type,
possessed of a strange avidity for oxygen, and capable of seizing
on it even when it is bound up in the molecules of water.
Considered as mere curiosities, these discoveries were
interesting, but aside from that they were of great theoretical
importance, because they showed the compound nature of some
familiar chemicals that had been regarded as elements. Several
other elementary earths met the same fate when subjected to the
electrical influence; the metals barium, calcium, and strontium
being thus discovered. Thereafter Davy always referred to the
supposed elementary substances (including oxygen, hydrogen, and
the rest) as "unde-compounded" bodies. These resist all present
efforts to decompose them, but how can one know what might not
happen were they subjected to an influence, perhaps some day to
be discovered, which exceeds the battery in power as the battery
exceeds the blowpipe?
Another and even more important theoretical result that flowed
from Davy's experiments during this first decade of the century
was the proof that no elementary substances other than hydrogen
and oxygen are produced when pure water is decomposed by the
electric current. It was early noticed by Davy and others that
when a strong current is passed through water, alkalies appear at
one pole of the battery and acids at the other, and this though
the water used were absolutely pure. This seemingly told of the
creation of elements--a transmutation but one step removed from
the creation of matter itself--under the influence of the new
"force." It was one of Davy's greatest triumphs to prove, in the
series of experiments recorded in his famous Bakerian lecture of
1806, that the alleged creation of elements did not take place,
the substances found at the poles of the battery having been
dissolved from the walls of the vessels in which the water
experimented upon had been placed. Thus the same implement which
had served to give a certain philosophical warrant to the fading
dreams of alchemy banished those dreams peremptorily from the
domain of present science.
"As early as 1800," writes Davy, "I had found that when separate
portions of distilled water, filling two glass tubes, connected
by moist bladders, or any moist animal or vegetable substances,
were submitted to the electrical action of the pile of Volta by
means of gold wires, a nitro-muriatic solution of gold appeared
in the tube containing the positive wire, or the wire
transmitting the electricity, and a solution of soda in the
opposite tube; but I soon ascertained that the muriatic acid owed
its existence to the animal or vegetable matters employed; for
when the same fibres of cotton were made use of in successive
experiments, and washed after every process in a weak solution of
nitric acid, the water in the apparatus containing them, though
acted on for a great length of time with a very strong power, at
last produced no effects upon nitrate of silver.
"In cases when I had procured much soda, the glass at its point
of contact with the wire seemed considerably corroded; and I was
confirmed in my idea of referring the production of the alkali
principally to this source, by finding that no fixed saline
matter could be obtained by electrifying distilled water in a
single agate cup from two points of platina with the Voltaic
battery.
"Mr. Sylvester, however, in a paper published in Mr. Nicholson's
journal for last August, states that though no fixed alkali or
muriatic acid appears when a single vessel is employed, yet that
they are both formed when two vessels are used. And to do away
with all objections with regard to vegetable substances or glass,
he conducted his process in a vessel made of baked tobacco-pipe
clay inserted in a crucible of platina. I have no doubt of the
correctness of his results; but the conclusion appears
objectionable. He conceives, that he obtained fixed alkali,
because the fluid after being heated and evaporated left a matter
that tinged turmeric brown, which would have happened had it been
lime, a substance that exists in considerable quantities in all
pipe-clay; and even allowing the presence of fixed alkali, the
materials employed for the manufacture of tobacco-pipes are not
at all such as to exclude the combinations of this substance.
"I resumed the inquiry; I procured small cylindrical cups of
agate of the capacity of about one-quarter of a cubic inch each.
They were boiled for some hours in distilled water, and a piece
of very white and transparent amianthus that had been treated in
the same way was made then to connect together; they were filled
with distilled water and exposed by means of two platina wires to
a current of electricity, from one hundred and fifty pairs of
plates of copper and zinc four inches square, made active by
means of solution of alum. After forty-eight hours the process
was examined: Paper tinged with litmus plunged into the tube
containing the transmitting or positive wire was immediately
strongly reddened. Paper colored by turmeric introduced into the
other tube had its color much deepened; the acid matter gave a
very slight degree of turgidness to solution of nitrate of soda.
The fluid that affected turmeric retained this property after
being strongly boiled; and it appeared more vivid as the quantity
became reduced by evaporation; carbonate of ammonia was mixed
with it, and the whole dried and exposed to a strong heat; a
minute quantity of white matter remained, which, as far as my
examinations could go, had the properties of carbonate of soda. I
compared it with similar minute portions of the pure carbonates
of potash, and similar minute portions of the pure carbonates of
potash and soda. It was not so deliquescent as the former of
these bodies, and it formed a salt with nitric acid, which, like
nitrate of soda, soon attracted moisture from a damp atmosphere
and became fluid.
"This result was unexpected, but it was far from convincing me
that the substances which were obtained were generated. In a
similar process with glass tubes, carried on under exactly the
same circumstances and for the same time, I obtained a quantity
of alkali which must have been more than twenty times greater,
but no traces of muriatic acid. There was much probability that
the agate contained some minute portion of saline matter, not
easily detected by chemical analysis, either in combination or
intimate cohesion in its pores. To determine this, I repeated
this a second, a third, and a fourth time. In the second
experiment turbidness was still produced by a solution of nitrate
of silver in the tube containing the acid, but it was less
distinct; in the third process it was barely perceptible; and in
the fourth process the two fluids remained perfectly clear after
the mixture. The quantity of alkaline matter diminished in every
operation; and in the last process, though the battery had been
kept in great activity for three days, the fluid possessed, in a
very slight degree, only the power of acting on paper tinged with
turmeric; but its alkaline property was very sensible to litmus
paper slightly reddened, which is a much more delicate test; and
after evaporation and the process by carbonate of ammonia, a
barely perceptible quantity of fixed alkali was still left. The
acid matter in the other tube was abundant; its taste was sour;
it smelled like water over which large quantities of nitrous gas
have been long kept; it did not effect solution of muriate of
barytes; and a drop of it placed upon a polished plate of silver
left, after evaporation, a black stain, precisely similar to that
produced by extremely diluted nitrous acid.
"After these results I could no longer doubt that some saline
matter existing in the agate tubes had been the source of the
acid matter capable of precipitating nitrate of silver and much
of the alkali. Four additional repetitions of the process,
however, convinced me that there was likewise some other cause
for the presence of this last substance; for it continued to
appear to the last in quantities sufficiently distinguishable,
and apparently equal in every case. I had used every precaution,
I had included the tube in glass vessels out of the reach of the
circulating air; all the acting materials had been repeatedly
washed with distilled water; and no part of them in contact with
the fluid had been touched by the fingers.
"The only substance that I could now conceive as furnishing the
fixed alkali was the water itself. This water appeared pure by
the tests of nitrate of silver and muriate of barytes; but potash
of soda, as is well known, rises in small quantities in rapid
distillation; and the New River water which I made use of
contains animal and vegetable impurities, which it was easy to
conceive might furnish neutral salts capable of being carried
over in vivid ebullition."[1] Further experiment proved the
correctness of this inference, and the last doubt as to the
origin of the puzzling chemical was dispelled.
Though the presence of the alkalies and acids in the water was
explained, however, their respective migrations to the negative
and positive poles of the battery remained to be accounted for.
Davy's classical explanation assumed that different elements
differ among themselves as to their electrical properties, some
being positively, others negatively, electrified. Electricity
and "chemical affinity," he said, apparently are manifestations
of the same force, acting in the one case on masses, in the other
on particles. Electro-positive particles unite with
electro-negative particles to form chemical compounds, in virtue
of the familiar principle that opposite electricities attract one
another. When compounds are decomposed by the battery, this
mutual attraction is overcome by the stronger attraction of the
poles of the battery itself.
This theory of binary composition of all chemical compounds,
through the union of electro-positive and electro-negative atoms
or molecules, was extended by Berzelius, and made the basis of
his famous system of theoretical chemistry. This theory held
that all inorganic compounds, however complex their composition,
are essentially composed of such binary combinations. For many
years this view enjoyed almost undisputed sway. It received what
seemed strong confirmation when Faraday showed the definite
connection between the amount of electricity employed and the
amount of decomposition produced in the so-called electrolyte.
But its claims were really much too comprehensive, as subsequent
discoveries proved.
ORGANIC CHEMISTRY AND THE IDEA OF THE MOLECULE
When Berzelius first promulgated his binary theory he was careful
to restrict its unmodified application to the compounds of the
inorganic world. At that time, and for a long time thereafter,
it was supposed that substances of organic nature had some
properties that kept them aloof from the domain of inorganic
chemistry. It was little doubted that a so-called "vital force"
operated here, replacing or modifying the action of ordinary
"chemical affinity." It was, indeed, admitted that organic
compounds are composed of familiar elements--chiefly carbon,
oxygen, hydrogen, and nitrogen; but these elements were supposed
to be united in ways that could not be imitated in the domain of
the non-living. It was regarded almost as an axiom of chemistry
that no organic compound whatever could be put together from its
elements--synthesized--in the laboratory. To effect the synthesis
of even the simplest organic compound, it was thought that the
"vital force" must be in operation.
Therefore a veritable sensation was created in the chemical world
when, in the year 1828, it was announced that the young German
chemist, Friedrich Wohler, formerly pupil of Berzelius, and
already known as a coming master, had actually synthesized the
well-known organic product urea in his laboratory at Sacrow. The
"exception which proves the rule" is something never heard of in
the domain of logical science. Natural law knows no exceptions.
So the synthesis of a single organic compound sufficed at a blow
to break down the chemical barrier which the imagination of the
fathers of the science had erected between animate and inanimate
nature. Thenceforth the philosophical chemist would regard the
plant and animal organisms as chemical laboratories in which
conditions are peculiarly favorable for building up complex
compounds of a few familiar elements, under the operation of
universal chemical laws. The chimera "vital force" could no
longer gain recognition in the domain of chemistry.
Now a wave of interest in organic chemistry swept over the
chemical world, and soon the study of carbon compounds became as
much the fashion as electrochemistry had been in the, preceding
generation.
Foremost among the workers who rendered this epoch of organic
chemistry memorable were Justus Liebig in Germany and Jean
Baptiste Andre Dumas in France, and their respective pupils,
Charles Frederic Gerhardt and Augustus Laurent. Wohler, too,
must be named in the same breath, as also must Louis Pasteur,
who, though somewhat younger than the others, came upon the scene
in time to take chief part in the most important of the
controversies that grew out of their labors.
Several years earlier than this the way had been paved for the
study of organic substances by Gay-Lussac's discovery, made in
1815, that a certain compound of carbon and nitrogen, which he
named cyanogen, has a peculiar degree of stability which enables
it to retain its identity and enter into chemical relations after
the manner of a simple body. A year later Ampere discovered that
nitrogen and hydrogen, when combined in certain proportions to
form what he called ammonium, have the same property. Berzelius
had seized upon this discovery of the compound radical, as it was
called, because it seemed to lend aid to his dualistic theory. He
conceived the idea that all organic compounds are binary unions
of various compound radicals with an atom of oxygen, announcing
this theory in 1818. Ten years later, Liebig and Wohler undertook
a joint investigation which resulted in proving that compound
radicals are indeed very abundant among organic substances. Thus
the theory of Berzelius seemed to be substantiated, and organic
chemistry came to be defined as the chemistry of compound
radicals.
But even in the day of its seeming triumph the dualistic theory
was destined to receive a rude shock. This came about through
the investigations of Dumas, who proved that in a certain organic
substance an atom of hydrogen may be removed and an atom of
chlorine substituted in its place without destroying the
integrity of the original compound--much as a child might
substitute one block for another in its play-house. Such a
substitution would be quite consistent with the dualistic theory,
were it not for the very essential fact that hydrogen is a
powerfully electro-positive element, while chlorine is as
strongly electro-negative. Hence the compound radical which
united successively with these two elements must itself be at one
time electro-positive, at another electro-negative--a seeming
inconsistency which threw the entire Berzelian theory into
disfavor.
In its place there was elaborated, chiefly through the efforts of
Laurent and Gerhardt, a conception of the molecule as a unitary
structure, built up through the aggregation of various atoms, in
accordance with "elective affinities" whose nature is not yet
understood A doctrine of "nuclei" and a doctrine of "types" of
molecular structure were much exploited, and, like the doctrine
of compound radicals, became useful as aids to memory and guides
for the analyst, indicating some of the plans of molecular
construction, though by no means penetrating the mysteries of
chemical affinity. They are classifications rather than
explanations of chemical unions. But at least they served an
important purpose in giving definiteness to the idea of a
molecular structure built of atoms as the basis of all
substances. Now at last the word molecule came to have a distinct
meaning, as distinct from "atom," in the minds of the generality
of chemists, as it had had for Avogadro a third of a century
before. Avogadro's hypothesis that there are equal numbers of
these molecules in equal volumes of gases, under fixed
conditions, was revived by Gerhardt, and a little later, under
the championship of Cannizzaro, was exalted to the plane of a
fixed law. Thenceforth the conception of the molecule was to be
as dominant a thought in chemistry as the idea of the atom had
become in a previous epoch.
CHEMICAL AFFINITY
Of course the atom itself was in no sense displaced, but
Avogadro's law soon made it plain that the atom had often usurped
territory that did not really belong to it. In many cases the
chemists had supposed themselves dealing with atoms as units
where the true unit was the molecule. In the case of elementary
gases, such as hydrogen and oxygen, for example, the law of equal
numbers of molecules in equal spaces made it clear that the atoms
do not exist isolated, as had been supposed. Since two volumes
of hydrogen unite with one volume of oxygen to form two volumes
of water vapor, the simplest mathematics show, in the light of
Avogadro's law, not only that each molecule of water must contain
two hydrogen atoms (a point previously in dispute), but that the
original molecules of hydrogen and oxygen must have been composed
in each case of two atoms---else how could one volume of oxygen
supply an atom for every molecule of two volumes of water?
What, then, does this imply? Why, that the elementary atom has
an avidity for other atoms, a longing for companionship, an
"affinity"--call it what you will--which is bound to be satisfied
if other atoms are in the neighborhood. Placed solely among
atoms of its own kind, the oxygen atom seizes on a fellow oxygen
atom, and in all their mad dancings these two mates cling
together--possibly revolving about each other in miniature
planetary orbits. Precisely the same thing occurs among the
hydrogen atoms. But now suppose the various pairs of oxygen atoms
come near other pairs of hydrogen atoms (under proper conditions
which need not detain us here), then each oxygen atom loses its
attachment for its fellow, and flings itself madly into the
circuit of one of the hydrogen couplets, and--presto!--there are
only two molecules for every three there were before, and free
oxygen and hydrogen have become water. The whole process, stated
in chemical phraseology, is summed up in the statement that under
the given conditions the oxygen atoms had a greater affinity for
the hydrogen atoms than for one another.
As chemists studied the actions of various kinds of atoms, in
regard to their unions with one another to form molecules, it
gradually dawned upon them that not all elements are satisfied
with the same number of companions. Some elements ask only one,
and refuse to take more; while others link themselves, when
occasion offers, with two, three, four, or more. Thus we saw that
oxygen forsook a single atom of its own kind and linked itself
with two atoms of hydrogen. Clearly, then, the oxygen atom, like
a creature with two hands, is able to clutch two other atoms.
But we have no proof that under any circumstances it could hold
more than two. Its affinities seem satisfied when it has two
bonds. But, on the other hand, the atom of nitrogen is able to
hold three atoms of hydrogen, and does so in the molecule of
ammonium (NH3); while the carbon atom can hold four atoms of
hydrogen or two atoms of oxygen.
Evidently, then, one atom is not always equivalent to another
atom of a different kind in combining powers. A recognition of
this fact by Frankland about 1852, and its further investigation
by others (notably A. Kekule and A. S. Couper), led to the
introduction of the word equivalent into chemical terminology in
a new sense, and in particular to an understanding of the
affinities or "valency" of different elements, which proved of
the most fundamental importance. Thus it was shown that, of the
four elements that enter most prominently into organic compounds,
hydrogen can link itself with only a single bond to any other
element--it has, so to speak, but a single hand with which to
grasp--while oxygen has capacity for two bonds, nitrogen for
three (possibly for five), and carbon for four. The words
monovalent, divalent, trivalent, tretrava-lent, etc., were coined
to express this most important fact, and the various elements
came to be known as monads, diads, triads, etc. Just why
different elements should differ thus in valency no one as yet
knows; it is an empirical fact that they do. And once the nature
of any element has been determined as regards its valency, a most
important insight into the possible behavior of that element has
been secured. Thus a consideration of the fact that hydrogen is
monovalent, while oxygen is divalent, makes it plain that we must
expect to find no more than three compounds of these two
elements--namely, H--O--(written HO by the chemist, and called
hydroxyl); H--O--H (H2O, or water), and H--O--O--H (H2O2, or
hydrogen peroxide). It will be observed that in the first of
these compounds the atom of oxygen stands, so to speak, with one
of its hands free, eagerly reaching out, therefore, for another
companion, and hence, in the language of chemistry, forming an
unstable compound. Again, in the third compound, though all hands
are clasped, yet one pair links oxygen with oxygen; and this also
must be an unstable union, since the avidity of an atom for its
own kind is relatively weak. Thus the well-known properties of
hydrogen peroxide are explained, its easy decomposition, and the
eagerness with which it seizes upon the elements of other
compounds.
But the molecule of water, on the other hand, has its atoms
arranged in a state of stable equilibrium, all their affinities
being satisfied. Each hydrogen atom has satisfied its own
affinity by clutching the oxygen atom; and the oxygen atom has
both its bonds satisfied by clutching back at the two hydrogen
atoms. Therefore the trio, linked in this close bond, have no
tendency to reach out for any other companion, nor, indeed, any
power to hold another should it thrust itself upon them. They
form a "stable" compound, which under all ordinary circumstances
will retain its identity as a molecule of water, even though the
physical mass of which it is a part changes its condition from a
solid to a gas from ice to vapor.
But a consideration of this condition of stable equilibrium in
the molecule at once suggests a new question: How can an
aggregation of atoms, having all their affinities satisfied, take
any further part in chemical reactions? Seemingly such a
molecule, whatever its physical properties, must be chemically
inert, incapable of any atomic readjustments. And so in point of
fact it is, so long as its component atoms cling to one another
unremittingly. But this, it appears, is precisely what the atoms
are little prone to do. It seems that they are fickle to the last
degree in their individual attachments, and are as prone to break
away from bondage as they are to enter into it. Thus the oxygen
atom which has just flung itself into the circuit of two hydrogen
atoms, the next moment flings itself free again and seeks new
companions. It is for all the world like the incessant change of
partners in a rollicking dance. This incessant dissolution and
reformation of molecules in a substance which as a whole remains
apparently unchanged was first fully appreciated by Ste.-Claire
Deville, and by him named dissociation. It is a process which
goes on much more actively in some compounds than in others, and
very much more actively under some physical conditions (such as
increase of temperature) than under others. But apparently no
substances at ordinary temperatures, and no temperature above the
absolute zero, are absolutely free from its disturbing influence.
Hence it is that molecules having all the valency of their atoms
fully satisfied do not lose their chemical activity--since each
atom is momentarily free in the exchange of partners, and may
seize upon different atoms from its former partners, if those it
prefers are at hand.
While, however, an appreciation of this ceaseless activity of the
atom is essential to a proper understanding of its chemical
efficiency, yet from another point of view the "saturated"
molecule--that is, the molecule whose atoms have their valency
all satisfied--may be thought of as a relatively fixed or stable
organism. Even though it may presently be torn down, it is for
the time being a completed structure; and a consideration of the
valency of its atoms gives the best clew that has hitherto been
obtainable as to the character of its architecture. How
important this matter of architecture of the molecule--of space
relations of the atoms--may be was demonstrated as long ago as
1823, when Liebig and Wohler proved, to the utter bewilderment of
the chemical world, that two substances may have precisely the
same chemical constitution--the same number and kind of
atoms--and yet differ utterly in physical properties. The word
isomerism was coined by Berzelius to express this anomalous
condition of things, which seemed to negative the most
fundamental truths of chemistry. Naming the condition by no
means explained it, but the fact was made clear that something
besides the mere number and kind of atoms is important in the
architecture of a molecule. It became certain that atoms are not
thrown together haphazard to build a molecule, any more than
bricks are thrown together at random to form a house.
How delicate may be the gradations of architectural design in
building a molecule was well illustrated about 1850, when Pasteur
discovered that some carbon compounds--as certain sugars--can
only be distinguished from one another, when in solution, by the
fact of their twisting or polarizing a ray of light to the left
or to the right, respectively. But no inkling of an explanation
of these strange variations of molecular structure came until the
discovery of the law of valency. Then much of the mystery was
cleared away; for it was plain that since each atom in a molecule
can hold to itself only a fixed number of other atoms, complex
molecules must have their atoms linked in definite chains or
groups. And it is equally plain that where the atoms are
numerous, the exact plan of grouping may sometimes be susceptible
of change without doing violence to the law of valency. It is in
such cases that isomerism is observed to occur.
By paying constant heed to this matter of the affinities,
chemists are able to make diagrammatic pictures of the plan of
architecture of any molecule whose composition is known. In the
simple molecule of water (H2O), for example, the two hydrogen
atoms must have released each other before they could join the
oxygen, and the manner of linking must apparently be that
represented in the graphic formula H--O--H. With molecules
composed of a large number of atoms, such graphic representation
of the scheme of linking is of course increasingly difficult,
yet, with the affinities for a guide, it is always possible. Of
course no one supposes that such a formula, written in a single
plane, can possibly represent the true architecture of the
molecule: it is at best suggestive or diagrammatic rather than
pictorial. Nevertheless, it affords hints as to the structure of
the molecule such as the fathers of chemistry would not have
thought it possible ever to attain.
PERIODICITY OF ATOMIC WEIGHTS
These utterly novel studies of molecular architecture may seem at
first sight to take from the atom much of its former prestige as
the all-important personage of the chemical world. Since so much
depends upon the mere position of the atoms, it may appear that
comparatively little depends upon the nature of the atoms
themselves. But such a view is incorrect, for on closer
consideration it will appear that at no time has the atom been
seen to renounce its peculiar personality. Within certain limits
the character of a molecule may be altered by changing the
positions of its atoms (just as different buildings may be
constructed of the same bricks), but these limits are sharply
defined, and it would be as impossible to exceed them as it would
be to build a stone building with bricks. From first to last the
brick remains a brick, whatever the style of architecture it
helps to construct; it never becomes a stone. And just as closely
does each atom retain its own peculiar properties, regardless of
its surroundings.
Thus, for example, the carbon atom may take part in the formation
at one time of a diamond, again of a piece of coal, and yet again
of a particle of sugar, of wood fibre, of animal tissue, or of a
gas in the atmosphere; but from first to last--from glass-cutting
gem to intangible gas--there is no demonstrable change whatever
in any single property of the atom itself. So far as we know, its
size, its weight, its capacity for vibration or rotation, and its
inherent affinities, remain absolutely unchanged throughout all
these varying fortunes of position and association. And the same
thing is true of every atom of all of the seventy-odd elementary
substances with which the modern chemist is acquainted. Every one
appears always to maintain its unique integrity, gaining nothing
and losing nothing.
All this being true, it would seem as if the position of the
Daltonian atom as a primordial bit of matter, indestructible and
non-transmutable, had been put to the test by the chemistry of
our century, and not found wanting. Since those early days of the
century when the electric battery performed its miracles and
seemingly reached its limitations in the hands of Davy, many new
elementary substances have been discovered, but no single element
has been displaced from its position as an undecomposable body.
Rather have the analyses of the chemist seemed to make it more
and more certain that all elementary atoms are in truth what John
Herschel called them, "manufactured articles"--primordial,
changeless, indestructible.
And yet, oddly enough, it has chanced that hand in hand with the
experiments leading to such a goal have gone other experiments
arid speculations of exactly the opposite tenor. In each
generation there have been chemists among the leaders of their
science who have refused to admit that the so-called elements are
really elements at all in any final sense, and who have sought
eagerly for proof which might warrant their scepticism. The first
bit of evidence tending to support this view was furnished by an
English physician, Dr. William Prout, who in 1815 called
attention to a curious relation to be observed between the atomic
weight of the various elements. Accepting the figures given by
the authorities of the time (notably Thomson and Berzelius), it
appeared that a strikingly large proportion of the atomic weights
were exact multiples of the weight of hydrogen, and that others
differed so slightly that errors of observation might explain the
discrepancy. Prout felt that it could not be accidental, and he
could think of no tenable explanation, unless it be that the
atoms of the various alleged elements are made up of different
fixed numbers of hydrogen atoms. Could it be that the one true
element--the one primal matter--is hydrogen, and that all other
forms of matter are but compounds of this original substance?
Prout advanced this startling idea at first tentatively, in an
anonymous publication; but afterwards he espoused it openly and
urged its tenability. Coming just after Davy's dissociation of
some supposed elements, the idea proved alluring, and for a time
gained such popularity that chemists were disposed to round out
the observed atomic weights of all elements into whole numbers.
But presently renewed determinations of the atomic weights seemed
to discountenance this practice, and Prout's alleged law fell
into disrepute. It was revived, however, about 1840, by Dumas,
whose great authority secured it a respectful hearing, and whose
careful redetermination of the weight of carbon, making it
exactly twelve times that of hydrogen, aided the cause.
Subsequently Stas, the pupil of Dumas, undertook a long series of
determinations of atomic weights, with the expectation of
confirming the Proutian hypothesis. But his results seemed to
disprove the hypothesis, for the atomic weights of many elements
differed from whole numbers by more, it was thought, than the
limits of error of the experiments. It was noteworthy, however,
that the confidence of Dumas was not shaken, though he was led to
modify the hypothesis, and, in accordance with previous
suggestions of Clark and of Marignac, to recognize as the
primordial element, not hydrogen itself, but an atom half the
weight, or even one-fourth the weight, of that of hydrogen, of
which primordial atom the hydrogen atom itself is compounded. But
even in this modified form the hypothesis found great opposition
from experimental observers.
In 1864, however, a novel relation between the weights of the
elements and their other characteristics was called to the
attention of chemists by Professor John A. R. Newlands, of
London, who had noticed that if the elements are arranged
serially in the numerical order of their atomic weights, there is
a curious recurrence of similar properties at intervals of eight
elements This so-called "law of octaves" attracted little
immediate attention, but the facts it connotes soon came under
the observation of other chemists, notably of Professors Gustav
Hinrichs in America, Dmitri Mendeleeff in Russia, and Lothar
Meyer in Germany. Mendeleeff gave the discovery fullest
expression, explicating it in 1869, under the title of "the
periodic law."
Though this early exposition of what has since been admitted to
be a most important discovery was very fully outlined, the
generality of chemists gave it little heed till a decade or so
later, when three new elements, gallium, scandium, and germanium,
were discovered, which, on being analyzed, were quite
unexpectedly found to fit into three gaps which Mendeleeff had
left in his periodic scale. In effect the periodic law had
enabled Mendeleeff to predicate the existence of the new elements
years before they were discovered. Surely a system that leads to
such results is no mere vagary. So very soon the periodic law
took its place as one of the most important generalizations of
chemical science.
This law of periodicity was put forward as an expression of
observed relations independent of hypothesis; but of course the
theoretical bearings of these facts could not be overlooked. As
Professor J. H. Gladstone has said, it forces upon us "the
conviction that the elements are not separate bodies created
without reference to one another, but that they have been
originally fashioned, or have been built up, from one another,
according to some general plan." It is but a short step from
that proposition to the Proutian hypothesis.
NEW WEAPONS--SPECTROSCOPE AND CAMERA
But the atomic weights are not alone in suggesting the compound
nature of the alleged elements. Evidence of a totally different
kind has contributed to the same end, from a source that could
hardly have been imagined when the Proutian hypothesis, was
formulated, through the tradition of a novel weapon to the
armamentarium of the chemist--the spectroscope. The perfection
of this instrument, in the hands of two German scientists, Gustav
Robert Kirchhoff and Robert Wilhelm Bunsen, came about through
the investigation, towards the middle of the century, of the
meaning of the dark lines which had been observed in the solar
spectrum by Fraunhofer as early as 1815, and by Wollaston a
decade earlier. It was suspected by Stokes and by Fox Talbot in
England, but first brought to demonstration by Kirchhoff and
Bunsen, that these lines, which were known to occupy definite
positions in the spectrum, are really indicative of particular
elementary substances. By means of the spectroscope, which is
essentially a magnifying lens attached to a prism of glass, it is
possible to locate the lines with great accuracy, and it was soon
shown that here was a new means of chemical analysis of the most
exquisite delicacy. It was found, for example, that the
spectroscope could detect the presence of a quantity of sodium so
infinitesimal as the one two-hundred-thousandth of a grain. But
what was even more important, the spectroscope put no limit upon
the distance of location of the substance it tested, provided
only that sufficient light came from it. The experiments it
recorded might be performed in the sun, or in the most distant
stars or nebulae; indeed, one of the earliest feats of the
instrument was to wrench from the sun the secret of his chemical
constitution.
To render the utility of the spectroscope complete, however, it
was necessary to link with it another new chemical
agency--namely, photography. This now familiar process is based
on the property of light to decompose certain unstable compounds
of silver, and thus alter their chemical composition. Davy and
Wedgwood barely escaped the discovery of the value of the
photographic method early in the nineteenth century. Their
successors quite overlooked it until about 1826, when Louis J. M.
Daguerre, the French chemist, took the matter in hand, and after
many years of experimentation brought it to relative perfection
in 1839, in which year the famous daguerreotype first brought the
matter to popular attention. In the same year Mr. Fox Talbot read
a paper on the subject before the Royal Society, and soon
afterwards the efforts of Herschel and numerous other natural
philosophers contributed to the advancement of the new method.
In 1843 Dr. John W. Draper, the famous English-American chemist
and physiologist, showed that by photography the Fraunhofer lines
in the solar spectrum might be mapped with absolute accuracy;
also proving that the silvered film revealed many lines invisible
to the unaided eye. The value of this method of observation was
recognized at once, and, as soon as the spectroscope was
perfected, the photographic method, in conjunction with its use,
became invaluable to the chemist. By this means comparisons of
spectra may be made with a degree of accuracy not otherwise
obtainable; and, in case of the stars, whole clusters of spectra
may be placed on record at a single observation.
As the examination of the sun and stars proceeded, chemists were
amazed or delighted, according to their various preconceptions,
to witness the proof that many familiar terrestrial elements are
to be found in the celestial bodies. But what perhaps surprised
them most was to observe the enormous preponderance in the
sidereal bodies of the element hydrogen. Not only are there vast
quantities of this element in the sun's atmosphere, but some
other suns appeared to show hydrogen lines almost exclusively in
their spectra. Presently it appeared that the stars of which
this is true are those white stars, such as Sirius, which had
been conjectured to be the hottest; whereas stars that are only
red-hot, like our sun, show also the vapors of many other
elements, including iron and other metals.
In 1878 Professor J. Norman Lockyer, in a paper before the Royal
Society, called attention to the possible significance of this
series of observations. He urged that the fact of the sun showing
fewer elements than are observed here on the cool earth, while
stars much hotter than the sun show chiefly one element, and that
one hydrogen, the lightest of known elements, seemed to give
color to the possibility that our alleged elements are really
compounds, which at the temperature of the hottest stars may be
decomposed into hydrogen, the latter "element" itself being also
doubtless a compound, which might be resolved under yet more
trying conditions.
Here, then, was what might be termed direct experimental evidence
for the hypothesis of Prout. Unfortunately, however, it is
evidence of a kind which only a few experts are competent to
discuss--so very delicate a matter is the spectral analysis of
the stars. What is still more unfortunate, the experts do not
agree among themselves as to the validity of Professor Lockyer's
conclusions. Some, like Professor Crookes, have accepted them
with acclaim, hailing Lockyer as "the Darwin of the inorganic
world," while others have sought a different explanation of the
facts he brings forward. As yet it cannot be said that the
controversy has been brought to final settlement. Still, it is
hardly to be doubted that now, since the periodic law has seemed
to join hands with the spectroscope, a belief in the compound
nature of the so-called elements is rapidly gaining ground among
chemists. More and more general becomes the belief that the
Daltonian atom is really a compound radical, and that back of the
seeming diversity of the alleged elements is a single form of
primordial matter. Indeed, in very recent months, direct
experimental evidence for this view has at last come to hand,
through the study of radio-active substances. In a later chapter
we shall have occasion to inquire how this came about.