A MODEL OF NATURE. 185 



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." 11 On the present occasion, however, we are occu- 

 pied 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. 



THE PROPERTIES OF ATOMS AM) MOLECULES. 



For it* 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 ploblems; but in spite of this there is little doubt 

 but that we have some fairly accurate knowledge of molecular motions 

 and magnitudes. 



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 neighboring molecules, in 1890, Sohncke b 

 detected evidence of such a change in films of average thickness 

 of 106 millionths of a millimeter (/^/*), and quite recently Rudolph 

 Weber found it in an oil film when the thickness was 115/<//. c 



Taking the mean of these numbers and combining the results of 

 different variants of the theory, we ma} 7 conclude that a film should 

 become unstable and tend to rupture spontaneously somewhere be- 

 tween the thicknesses of 110 and 55 /</<, and Professor Remold and I 

 found by experiment that this instability is actually exhibited between 

 the thickness of 90 and 45 MF- a There can therefore be little doubt 

 that the first approach to molecular magnitude is signaled when the 

 thickness of a film is somewhat less than 100 /</*, or four millionths of 

 an inch. 



Thirteen years ago I had the honor of laying before the Chemical 



a For the most recent account of this subject, see an article on "Bodies smaller than 

 atoms," by Prof. J. J. Thomson, in the Popular Science Monthly (The Science Press), 

 August, 1901. [Reprinted in the present Smithsonian Report.] 



MVied. Ann., 1890, XL, pp. 345-355. 



"Annalen der Physik, 1901, IV, pp. 706-721. 



'Phil. Trans., 1893, 184, pp. 505-529. 



