TRANSACTIONS OF THE SECTIONS. 5 



tind was commonly supposed to admit the existence only of atoms and void, to the 

 exclusion of any other basis of things from the universe. 



In many physical reasonings and mathematical calculations we are accustomed 

 to argue as if such substances as air, water, or metal, which appear to our senses 

 uniform and continuous, were strictly and mathematically uniform and continuous. 



"We know that we can divide a pint of water into many millions of portions, 

 each of which is as fully endowed with all the properties of water as the whole 

 pint was ; and it seems only natural to conclude that we might go on subdividing 

 the water for ever, just as we can never come to a limit in subdividing the space 

 in which it is contained. We have heard how Faraday divided a gi-ain of gold into 

 an inconceivable number of separate particles, and we may see Dr. TjTidall produce 

 from a mere suspicion of nitrite of butyle an immense cloud, the minute visible 

 portion of which is still cloud, and therefore must contain many molecules of nitrite 

 of butyle. 



But evidence from different and independent sources is now crowding in upon us 

 which compels us to admit that if we could push the process of subdivision still 

 further we should come to a limit, because each portion would then contain only 

 one molecule, an individual body, one and indivisible, unalterable by any power in 

 nature. 



Even in our ordinary experiments on very finely diA'ided matter we find that the 

 substance is beginning to lose the properties which it exhibits when in a large mass, 

 and that effects depending on the individual action of molecules are beginning to 

 become prominent. 



The study of these phenomena is at present the path which leads to the develop- 

 ment of molecular science. 



That superficial tension of liquids which is called capillary attraction is one of 

 these phenomena. Another important class of phenomena are those which are due 

 to that motion of agitation by which the molecules of a liquid or gas are continu- 

 ally working their way from one place to another, and continually changing their 

 course, like people hustled in a crowd. 



On this depends the rate of diffusion of gases and liquids through each other, 

 to the study of which, as one of tlie keys of molecular science, that unwearied in- 

 quh-er into nature's secrets, the late Prof. Graham, devoted such arduous labour. 



The rate of electrolytic conduction is, according to Wiedemann's theory, 

 influenced by the same cause ; and the conduction of heat in fluids depends pro- 

 bably on the same kind of action. In the case of gases, a molecular theoiy has 

 been developed by Clausius and others, capable of mathematical treatment, and 

 eubjected to experimental investigation; and by this theory nearly every known 

 mechanical property of gases has been explained on dynamical principles ; so that 

 the properties of individual gaseous molecules are in a fair way to become objects 

 of scientific research. 



Now Mr. Stoney has pointed out * that the numerical results of experiments on 

 gases render it probable that the mean distance of their particles at the ordinary 

 temperature and pressure is a quantity of the same order of magnitude as a mil- 

 lionth of a millimetre, and Sir William Thomson has since t shown, by several 

 independent lines of argument, drawn from phenomena so different in themselves 

 as the electrification of metals by contact, the tension of soap-bubbles, and the 

 fidction of air, that in ordinary solids and liquids the average distance between 

 contiguous molecules is less than the hundred-milUouth, and greater than the two- 

 thousand-millionth of a centimetre. 



These, of course, are exceedingly rough estimates, for they are derived from 

 measurements some of which are still confessedly very rough ; but if, at the present 

 time, we can form even a rough plan for arriving at results of this kind, we may hope 

 that, as our means of experimental inquiry become more accurate and more varied, 

 our conception of a molecule will become more definite, so that we may be able 

 at no distant period to estimate its weight with a gi-eater degree of precision. 



A theory, which Sir W. Thomson has founded on Helmholtz's splendid hydro- 

 dynamical theorems, seeks for the properties of molecules in the ring-vortices of a 

 uniform, frictionless, incompressible fluid. Such whirling rings may be seen when 

 * PhU. Mag. Aug. 1868. | Nature, March 31, 1870. 



