ADDRESS. 19 



It is, however, from the theory of ions that the most far-reaching 

 speculations of science have recently received unexpected support. The 

 dream that matter of all kinds will some day be proved to be funda- 

 mentally the same has survived many shocks. The opinion is consistent 

 with the great generalisation that the properties of elements are a 

 periodic function of their atomic weights. Sir Norman Lockyer has 

 long been a prominent exponent of the view that the spectra of the 

 stars indicate the reduction of our so-called elements to simpler forms, 

 and now Professor J. J. Thomson believes that we can break off from an 

 atom a part, the mass of which is not more than one thousandth of the 

 whole, and that these corpuscles, as he has named them, are the carriers 

 of the negative charge in an electric current. If atoms are thus 

 complex, not only is the a priori probability increased that the different 

 structures which we call elements may all be built of similar bricks, but 

 the discovery by Lenard that the ease with which the corpuscles 

 penetrate different bodies depends only on the density of the obstacles, 

 and not on their chemical constitution, is held by Professor Thomson to 

 be ' a strong confirmation of the view that the atoms of the elementary 

 substances are made up of simpler parts, all of which are alike.' ' On 

 the present occasion, however, we are occupied rather with the foundations 

 than with these ultimate ramifications of the atomic theory ; and having 

 shown how wide its range is, I must, to a certain extent, retrace my steps 

 and return to the main line of my argument. 



Tlie Properties of Atoms and Molecules. 



For if it be granted that the evidence that matter is coarse-grained 

 and is formed of separate atoms and molecules is too strong to be resisted, 

 it may still be contended that we can know little or nothing of the sizes 

 and properties of the molecules. 



It must be admitted that though the fundamental postulates are 

 always the same, different aspects of the theory, which have not in all 

 cases been successfully combined, have to be developed when it is applied 

 to different problems ; but in spite of this there is little doubt that we 

 have some fairly accurate knowledge of molecular motions and magni- 

 tudes. 



If a liquid is stretched into a very thin film, such as a soap bubble, 

 we should expect indications of a change in its properties when the 

 thickness of the film is not a very large multiple of the average distance 

 between two neighbouring molecules. In 1890 Sohncke - detected evi- 

 dence of such a change in films of the average thickness of 106 million ths 



' For the most recent account of this subject see an article on ' Bodies smaller 

 than Atoms,' by Professor J. J. Thomson in the Popular Scirnir Montldij (Tlie 

 Science Press), August 1901. 



• Wied. Ann., 189U, xl. pp. 345-355. 



C2 



