496 
NATURE 
| SEPTEMBER 18, 1902 
the theory quite explicitly, and then the anomalous condition of 
things will be removed of a theory being in universal use without 
its truth being freely and openly admitted. For the sake of 
clearness, it is convenient to restrict the term ‘‘atomic hypo- 
thesis” to the old metaphysical view of the discontinuity of 
matter whilst applying the term ‘‘ atomic theory” to the current 
elaborated form of the Daltonian theory; this distinction is 
adhered to in the present Address. 
In the peroration to his admirable discourse upon atomic 
weights or masses delivered before the Chemical Society in 1892 
as the Stas Memorial Lecture, Prof. Mallet, F.R.S., said: 
“* By the chemist at his balance the arm of reason is directed into 
those regions of almost inconceivable minuteness, which lie as 
far beyond the reach of the most powerful microscope as that 
carries us beyond the reach of the naked eye, quite as impres- 
sively as that same arm is stretched forth by the astronomer at 
his divided circle to reach and to weigh the mighty planets that 
shine in the remotest regions of our solar system.” On two 
occasions I have heard the same comparison between the chemist 
and the astronomer made by Lord Kelvin when he was in the 
company of chemists ; and undoubtedly both these high authori- 
ties have only then expressed the general view as to the nature 
of the domain of the chemist. Yet I venture to question whether 
there is anything in the ways and work of the chemist to support 
such a view and give point to Mallet and Kelvin’s comparison. 
If, indeed, chemistry is a science which rests upon the atomic 
hypothesis and, therefore, would cease to exist in the form into 
which it has developed should matter prove to be continuous 
and not discrete, nothing can be said against the view that it is 
a science of the minute. But I am sure there can be no one 
ready to maintain that, if the hypothesis of the atomic 
constitution of substances were an unfounded one, the 
atomic theory would have been a discovery of no great im- 
portance; and Dalton himself, instead of being the founder 
of the chemistry of to-day, have been little more than the 
discoverer of the law of multiple proportions. If that cannot be 
maintained, what, then, becomes of this conception of chemistry 
as dealing with the minute? So far as comparison can be made 
between the operations of the astronomer and the chemist, it is 
the former and not the latter who, as a matter of fact, deals 
with the almost infinitely minute. For if, indeed, the chemist 
often works upon comparatively small amounts of substances, 
and, consequently, with very sensitive balances, that is, as we 
all know, only for reasons of economy of time, materials and 
apparatus ; otherwise he works on the largest possible scale, 
with the object of attaining to the highest degree of accuracy 
and perfection. The astronomer, on the other hand, has, per- 
force, to deal with the smallest visible things in nature, the 
nearest approach there is to geometrical points, those fixed 
points of light in the heavens which are only known through 
scientific investigation to be other than what they seem to be. 
It is, therefore, only as interpreted by the atomic hypothesis 
that chemistry can be said to deal with the minute. 
When the atomic theory is expounded in the usual way it is 
commonly and correctly stated that, on the assumption that 
substances consist of minute indivisible particles having weights 
or masses bearing the ratios of the combining numbers assigned 
to them, the laws of chemical combination by weight necessarily 
follow, and are thereby explained. But then the converse is 
not true—that because chemical combination obeys the well- 
known laws, substances consist of discrete particles. Nor does 
the assumption of the truth of the atomic hypothesis afford any 
real explanation of the facts expressed by the laws of chemical 
combination, or more comprehensively by the atomic theory, 
when that theory is given in non-hypothetical terms. It is just 
as difficult to see why the atoms should possess the weights on 
chemical grounds assigned to them as to see why substances 
interact in the proportions that they do; that they do do so is, in 
either case, an ultimate fact, for which no explanation has pre- 
sented itself. The atomic hypothesis masks this ignorance and 
deadens inquisitiveness. Notwithstanding all this, which is 
incontrovertible, it is certainly a common opinion that in 
chemistry we investigate the minute and intimate constitution 
of things. 
But if, after all, chemistry does not deal with the minute, or, 
rather, if it has no concern with the magnitude of single bodies 
or their molecules ; if the atomic hypothesis is not the founda- 
tion of, or necessary to, the atomic theory, then it is certainly 
most desirable and important that the theory of chemistry, which, 
with all its modern developments, I take to be indisputably the 
NO. 1716, vor. 66] 
atomic theory of Dalton, should be held and expounded without 
any reference to the physical constitution of matter, in so far as 
that remains unknown. The opinion that chemical theory 
should be developed without reference to the atomic hypothesis 
has indeed all along been held by many eminent chemists ; but 
then the dilemma appears to have presented itself to them, that 
either the atomic hypothesis must be granted or the atomic 
theory must be dispensed with, since it falls with the hypothesis. 
That dilemma I do not recognise, and the practice of chemists 
shows beyond doubt that it is always ignored. Investigators use 
the theory whether they admit it or not; teachers of the science 
find it indispensable to their task, however much they may 
deprecate, and rightly so, unreserved acceptance of the atomic 
hypothesis as true. 
Refusing to commit themselves to belief in the hypothesis, 
chemists have thought from the first to escape the adoption of 
the atomic theory by putting Dalton’s discovery into something 
like these words: Numbers, called proportional or combining 
numbers, can be assigned to the chemical elements—one to each 
—which will express all the ratios of the weights or masses in 
which substances interact and combine together. Perhaps the 
atomic theory is here successfully set aside by expressing what is 
an actuality as an unaccounted-for possibility. But then those 
who use any such mode of expressing the facts, without reference 
to the theory, never fail also to adopt the doctrine of equivalents, 
and thus, by this double act, implicitly give in their adherence 
to the theory. 
Divested of all reference to the physical constitution of matter, 
the atomic theory is that the quantities of substances which 
interact in single chemical changes are equal to one another— 
as truly equal in one way as equal masses are in another—and, 
therefore, that chemical interaction is a measure of quantity of 
unlike substanees, distinct from and independent of dynamical 
Or mass measurement. 
Dalton, indeed, did not express himself in any such terms, his 
mind being fully possessed with the ancient and current belief 
upon which he: framed his theory that substances are made up 
of minute, discrete particles. But it is clear enough that his 
theory was that of the existence of another order of equality 
between substances than that of weight. Up to his time, the 
weight or mass of every ultimate particle of any substance what- 
ever appears to have been assumed to be the same, the atoms 
being alike in every way. That assumption is still made by 
many thinkers, chemists among them ; we meet it, for example, 
in the different forms of the hypothesis that the elements are 
all, in some way, physically compounded of a universal and only 
true element, as in Prout’s hypothesis. Dalton saw things 
differently, and recognised that, on the assumption of substances 
being constituted of particles which never subdivide, weight or 
mass cannot be the same for every such particle, except in the case 
of those of any one simple substance. Therefore, having given 
some numbers showing what he believed to be the respective 
weights of the atoms of several simple substances, taking that of 
hydrogen as of unit-weight, he proceeded at once to invent 
symbols for these atoms to indicate, not only their distinctness 
in kind, but above all things their indivisibility and their equality, 
properties which the use of their atomic numbers would have in- 
advertently concealed or even apparently denied, and could 
never have expressed or connoted. 
It was only in this immediate invention and use of chemical 
symbols that Dalton’s conception found clear expression ; and 
again it is by the universal adoption of such symbols that 
chemists have shown their real acceptance of the atomic theory, 
even while displaying, not infrequently, their scepticism as to its 
truth. The replacement by Berzelius of Dalton’s marked circles 
for atomic symbols by letters which should recall the names of 
the substances was in a way a great improvement, but it has 
had the serious consequence of causing chemical symbols to be 
usually first brought under notice merely as serviceable abbre- 
viations for the names of the elements, and only then described 
as representing their atomic quantities. Now, evidently, what 
the character used as symbol shall be is, theoretically considered, 
but a petty detail ; the vital point is what the character symbol- 
ises, and that is the atom. It does not symbolise the name ; it 
only indicates that and recalls it. It may be said, indeed, to re- 
present the atomic number, since it stands in place of it ; but it 
is made to do so only in order that we may for the time forget 
this number and have in mind the integral character of the atom. 
It is not the 4006 parts of sodium hydroxide and 8097 parts of 
hydrobromic acid, or approximately twice as much of the latter 
