TRANSACTIONS OP THE SECTIONS. 45 



bodies for heat rather than in committing oui-selves to assertions incapable of proof 

 by chemical means. 



Agreeing in the main with the opinion of the last two chemists, and believing 

 that we must carefully distinguish between fact and theory, I would remind you 

 that Dalton's discovery of multiple and reciprocal proportions (I use Dr. Odling's 

 word), as well aa the differences which we now acknowledge in the power of 

 hydrogen-replacement in hydrochloric acid, water, ammonia, and marsh-gas, are 

 facts, whilst the explanation upon the assumption of atoms is, as far as chemistry 

 has yet advanced, a theory. If, however, the existence of atoms cannot be proved 

 by chemical phenomena, we must remember that the assumption of the atomic 

 theoiy explains chemical facts as the undulatory theory gives a clear view of the 

 phenomena of light; thus, for instance, one of the most important facts and 

 relations of modern chemistry which it appears difficult, if not impossible, to 

 explain without the assumption of atoms is that of. Isomerism. How otherwise 

 than by a different arrangement of the single constituent particles are we to account 

 for several distinct substances in which the proportions of carbon, hydrogen, and 

 oxygen are the same ?; Why, for instance, should 48 parts, by weight, of carbon, 10 

 of hydrogen, and 16 of oxygen, united together, be capable of existing as three 

 different chemical substances, unless we presuppose a difterent statical arrangement 

 of the parts by which these differences in the deportment of the whole are rendered 

 possible ? 



Although chemistry appears not to be able to give us positive information as to 

 whether matter is infinitely divisible, and therefore continuous, or whether it con- 

 sists of atoms and is discontinuous, we are in some degree assisted in this inquiry 

 by deductions from physical phenomena recently pomtcd out by the genius of 

 Sir Wm. Thomson. This philosopher, arguing from four distinct classes of 

 physical phenomena, not only comes to the conclusion that matter is discontinuous, 

 and therefore that atoms and molecules do exist, but he even attempts to form an 

 idea of the size of these molecules ; and he states that in any ordinary liquid or 

 transparent or seemingly opaque solid, the mean distance between the centres of 

 contiguous molecules is less than the hundi-ed-miUionth, and greater than the two- 

 thousand-miliionth of a centimetre. Or, to form a conception of this coarse-grained- 

 ness, imagine a raindrop, or globe of glass as large as a pea, to be magnified up to 

 the size of the earth, each constituent molecule being magnified in the same propor- 

 tion, the magnified structure would be coarser-grained than a heap of small shot, 

 but probably less coarse-grained than a heap of cricket-balls. 



There is, however, another class of physical considerations which render the 

 existence of indivisible particles more than likely. I refer to the mechanical 

 theory of gases, by means of which, thanks to the labours of eminent English and 

 German philosophers, all the physical properties of gases (their equal expansion 

 by heat — the laws of diffusion — the laws of alteration of volume imder pressure) 

 can be shovra to follow from the simple laws of mechanical motion. This theory, 

 however, presupposes the existence of molecules ; and in this direction again we 

 find confirmation of the real existence of Dalton's atoms. Indeed it has been 

 proved that the average velocity with which the particles of oxygen, nitrogen, or 

 common air are continually projected forwards amomits, at the ordinary atmospheric 

 pressure, to 50,000 centimetres per second, whilst the average number of impacts 

 of each of these molecules is 5000 millions per second. 



The mention of the molecular motions of gases will recall to the minds of all 

 present the great loss which English science has this year sustained in the death 

 of the discoverer of the laws of gaseous diffusion. Throughout his life Graham's 

 aim was the advancement of our knowledge in the special subject of the molecular 

 properties of gases. With this intent he imceasingly laboured up to the moment of 

 his death, in spite of failing health and pressure of official business, unfolding for 

 posterity some of the most difficult, as well as the most interesting, secrets of 

 nature in this branch of our science. " What do you think," he writes to Hofmann, 

 " of metallic hydrogen, a white magnetic metal ? " and yet now through his labours 

 the fact of the condensation of hydrogen in the solid state by metallic palladium, 

 and to a less extent by other metals, has become familiar to all of us. Then, again, 

 I would remind you of Graham's recent discovery of the occlusion of hydrogen 



