||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.