4, REPORT—1874. 
of scientific thought is setting strongly in that direction. The constant tendency of 
scientific thought is, as I have said, to increase the number of those phenomena 
which are regarded as mere varieties of motion. Sound—that we have placed on the 
list long since. Light, though here our conclusions are more hypothetical, we have 
also long regarded as belonging to the same category ; and Heat may now be fairly 
added ; and we have almost learned, under the guidance of Professor Williamson, 
to regard chemical combination asa phenomenon of the same kind. All these phe- 
nomena (of sound, of light, of heat, and perhaps even of chemical combination) 
we now regard as produced by the movements of systems of exceedingly small par- 
ticles—whether of known particles, as in the case of sound, or of the hypothetical 
zther, as in the case of light ; and a science which proposes to itself the mathe- 
matical discussion of the laws which govern the movements of such systems can 
hardly fail to play an important part in the future history of physical science. I 
shall not then, I hope, be thought to misemploy the time of the Section by offering 
some observations on the science of molecular dynamics. ‘ 
When we have to deal with a science which professes to be more than a mathe- 
matical abstraction—a science which assumes to itself the function of representing, 
with at least approximate truth, the realities of nature—our first question will 
naturally be, What is the basis on which it rests? Is it built upon a pure hypo- 
thesis, not derived from experiment, but seeking to justify its claim to reality by 
the truth of the results which may be deduced from it ? 
The word “molecule,” as Prof. Clerk Maxwell has told us, is modern, embody- 
ing an idea derived from modern chemistry. It denotes a material particle so small 
as to be incapable of subdivision into parts similar in their nature to itself. Thus 
a drop of water may be divided into smaller drops, each of which is also water; but 
a molecule of water is regarded as incapable of such division. Not that we regard 
it as absolutely indivisible; but we assume that a further division, could it be 
effected, would produce molecules, not of water, but of its component gases, hydro- 
gen and oxygen. . 
Now this conception of a moleeule undoubtedly involves an hypothesis. Are 
there such ultimate particles of matter, not only resisting all the a viding forces 
which we can command, but absolutely indivisible, by any force, into particles 
similar to each other, or perhaps into particles of any kind? Or are we to suppose 
that, if we had instruments of sufficient delicacy, the process of division might be 
carried on without limit? Experiment gives us no means of deciding between 
these alternatives; and if the exigencies of our method of investigation force us to 
make a decision, we can make it only by an hypothesis, But we may fairly ask, 
Does the logic of molecular dynamics absolutely require this decision? And on this 
point I wish to offer one or two remarks. When we propose to determine the 
motion of a body, solid or fluid, we ought, as indeed in all scientific problems, to 
form in the first place a clear conception of the meaning of the question which we 
propose to cia We wish to discover the laws which govern the motion—of 
what? Not certainly of the body taken as a whole. That is, no doubt, part of 
the information which we seek, but a very small part of it. When we have learned 
to determine by a fixed mathematical rule, or formula as we generally call it, the 
position occupied at any instant by the centre of gravity of the body and by its 
principal axes, we have learned something, but the investigation is far from being 
complete. There are, as you know, large classes of movements of which such 
knowledge would tell us nothing. Thus, to take a familiar instance, you see aman 
(to use our ordinary language) “sitting quiet.” He is at rest, so far as the moye- 
ment of the body, taken as a whole, is concerned. He is neither turning on his 
chair nor walking about the room ; and yet there is probably not a single particle 
of his body which is absolutely quiescent. You see, then, how ignorant we are of 
the vital movements of the human body, if we know only that the individual is 
“sitting quiet.” 
But suppose that we push the inquiry a little further and propose to investigate 
the motion of the blood. We obtain an answer to this question in one sense by 
determining the rate at which the blood, taken as a whole, is moving—that is to 
say, suppose the number of ounces of blood which pass through the mitral valve 
in the space of one minute; but having learned this, we are still very far from 
