ADDRESS TO THE MATHEMATICAL AND PHYSICAL SECTIONS 



.-till further we should come to a limit, because each portion would then contain 

 onlv one molecule, an individual body, one and indivisible, unalterable by any 

 power in nature. 



Kven in our ordinary experiments on very finely divided 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 mole- 

 cules are beginning to become prominent. 



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

 development 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 continually 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 the keys of molecular science, that 

 unwearied inquirer 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 

 probably on the same kind of action. In the case of gases, a molecular theory 

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

 and subjected 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 

 Income 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 millionth of a millimetre, and Sir William Thomson has sincet shewn, 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 friction of air, that in ordinary solids and liquids the average 

 distance between contiguous molecules is less than the hundred-millionth, and 

 greater than the two-thousand-millionth of a centimetre. 



* Phil. Mag. Aug. 1868. t Nature, March 31, 1870. 



