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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 WEIGHTSThese 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 CAMERABut the atomic weights are not alone in suggesting the compound
nature of the alleged elements. Evidence of a totally different
kind has
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