| Sept. 25 
, 1873] 
MOLECULES* ~ 
AN atom is a body which cannot be cut in two. <A molecule 
~ is the smallest possible portion of a particular substance. 
No one has ever seen or handled a single molecule. Mole- 
cular science, therzfore, is one of those branches of study which 
deal with things invisible and imperceptible by our senses, and 
which cannot be subjected to direct experiment. 
The mind of man has perplexed itself with many hard ques- 
tions. Is space infinite, and if so in what sense? Is the mate- 
rial world infinite in extent, and are all places within that extent 
equally full of matter? Do atoms exist, or is matter infinitely 
divisible ? 
The discussion of questions of this kind has been going on 
ever since men began to reason, and to each of us, assoon as we 
obtain the use of our faculties, the same old questions arise as 
fresh as ever. They form as essential a part of the science of the 
nineteenth century of our era, as of that of the fifth century 
before it. 
We do not know much about the science organisation of 
Thrace twenty-two centuries ago, or of the machinery then em- 
ployed for diffusing an interest in physical research. There were 
men, however, in those days, who devoted their lives to the 
pursuit of knowledge with an ardour worthy of the most distin- 
guished members of the British Association ; and the lectures in 
which Democritus explained the atomic theory to his fellow- 
citizens of Abdera realised, not in golden opinions only, but in 
golden talents, a sum hardly equalled even in America. 
To another very eminent philosopher, Anaxagoras, best known 
to the world as the teacher of Socrates, we are indebted for the 
most important service to the atomi¢ theory, which, after its 
statement by Democritus, remained to be done. Anaxagoras, 
in fact, stated a theory which so exactly contradicts the atomic 
theory of Democritus that the truth or falsehood of the one theory 
implies the falsehood or truth of the other. The question of the 
existence or non-existence of atoms cannot be presented to us 
this evening with greater clearness than in the alternative theories 
of thes: two philosophers. 
Take any portion of matter, say a drop of water, and observe its 
properties. Like every other portion of matter we have ever seen, 
it is divisible. Divide it in two, each portion appears to retain 
all the properties of the original drop, and among others that of 
being divisible. The parts are similar to the whole in every 
respect except in absolute size. 
Now go on repeating the process of division till the separate 
ortions of water are so small that we can no longer perceive or 
andle them. Still we have no doubt that the sub-division 
might be carried further, if our senses were more acute and our 
instruments more delicate. Thus far all are agreed, but now 
the question arises, Can this sub-division be repeated for ever ? 
According to Democritus and the atomic school, we must 
answer in the negative. After a certain number of sub-divisions, 
the drop’ would be divided into a number of parts each of which 
is incapable of further sub-division, We should thus, in imagi- 
nation, arrive at the atom, which, as its name literally signifies, 
cannot be cutin two. This is the atomic doctrine of Demo- 
critus, Epicurus, and Lucretius, and, I may add, of your lec- 
turer. 
According to Anaxagoras, on the other hand, the parts into 
which the drop is divided, are in all respects similar to the whole 
drop, the mere size of a body counting for nothing as regards 
the nature of its substance. Hence if the whole drop is divisible, 
so are its parts down to the minutest sub-divisions, and that with- 
out end. 
The essence of the doctrine of Anaxagoras is that the parts of 
a body are in all respects similar to the whole. It was therefore 
called the doctrine of Homoiomereia. Anaxagoras did not of 
course assert this of the parts of organised bodies such as men 
and animals, but he maintained that those inorganic substances 
which appear to us homogeneous are really so, and that the uni- 
versal experience of mankind testifies that every material body, 
without exception, is divisible. 
The doctrine of atoms and that of homogeneity are thus in 
direct contradiction. 
But we must now go on to molecules. Molecule is a modern 
word, It does not occur in Fohnson’s Dictionary. The ideas 
it embodies are those belonging to modern chemistry. 
_A drop of water, to return to our former example, may be 
divided into a certain number, and no more, of portions similar 
ae spivered: before the British Association at Bradford, by Prot. 
NATURE 
437 
to each other. Each of these the modern chemist calls a mole- 
cule >f water. But itis by no means an atom, for it contains 
two different substances, oxygen and hydrogen, and by a certain 
process the molecule may be actually divided into two parts, one 
consisting of oxygen and the other of hydrogen. According to 
the received doctrine, in each molecule of water there are two 
molecules of hydrogen and one of oxygen. Whether these are 
or are not ultimate atoms I shall not attempt to decide. 
We now see what a molecule is, as distinguished from an 
atom. . 
A molecuie of a substance is a small body such that if, on the 
one hand, a number of similar molecules were assembled to- 
gether they would form a mass of that substance, while on the 
other hand, if any portion of this molecule were removed, it 
would no longer be able, along with an assemblage of other 
molecules similarly treated, to make up a mass of the original 
substance. 
Every substance, simple or compound, has its own molecule. 
If this molecule be divided, its parts are molecules of a differeat 
substance or substances from that of which the whole is a mole- 
cule. An atom, if there is such a thing, must be a molecule of 
an elementary substance. Since, therefore, every molecule is 
not an atom, but every atom is a molecule, I shall use the word 
molecule as the more general term, 
I have no intention of taking up your time by expounding the 
doctrines of modern chemistry with respect to the molecules -of 
different substances. It is not the special but the universal inte- 
rest of molecular science which encourages me to address you. 
It is not because we happen to be chemists or physicists or spe- 
cialists of any kind that we are attracted towards this centre of 
all material existence, but because we all belong to a race en- 
dowed with faculties which urge us on to search deep and ever 
deeper into the nature of things. 
We find that now, as in the days of the earliest physical specu- 
lations, all physical researches appear to converge towards the 
same point, and every inquirer, as he looks forward into 
the dim region towards which the path of discovery is leading 
him, sees, each according to his sight, the vision of the same 
quest. 
One may see the atom as a material point, invested and sur- : 
rounded by potential forces. Another szes no garment of force, 
but only the bare and utter hardness of mere impenetrability. 
But though many a speculator, as he has seen the vision recede 
before him into the innermost sanctuary of the inconceivably 
little, has had to confess that the quest was not for him, and 
though philosophers in every age have been exhorting each other 
to direct their minds to some more useful and attainable aim, 
each generation, from the earliest dawn of science to the present 
time, has contributed a due proportion of its ablest intellects to 
the quest of the ultimate atom. 
Our business this evening is to describe some researches in 
molecular science, and in particular to place before you any de- 
finite information which has been obtained respecting the mole- 
cules themselves. The old atomic theory, as described by Lucretius 
and revived in modern times, asserts that the molecules of all 
bodies are in motion, even when the body itself appears to be at 
rest. These motions of molecules are in the case of solid bodies 
confined within so narrow a range that even with our best 
microscopes we cannot detect that they alter their places at all. 
In liquids and gases, however, the molecules are not confined 
within any definite limits, but work their way through the 
whole mass, even when that mass is not disturbed by any visible 
motion. 
‘This process of diffusion, as it is called, which goes on in gases ~ 
and liquids and even in some solids, can be subjected to experi- 
ment, and forms one of the most convincing proofs of the motion 
of molecules. 
Now the recent progress of molecular science began with the 
study of the mechanical effect of the impact of these moving 
molecules when they strike against any solid body. Of course 
these flying molecules must beat against whatever is placed 
among them, and the constant succession of these strokes is, ac- 
cording to our theory, the sole cause of what is called the pressure 
of air and other gases. 
This appears to have been first suspected by Daniel Bernoulli, 
but he had not the means which we now have of verifying the 
theory. The same theory was afterwards brought forward 
independently by Lesage, of Geneva, who, however, devoted 
most of his labour to the explanation of gravitation by the im- 
pact of atoms, Then Herapath, in his ‘‘ Mathematical Physics,” 
4 
