172 PROGRESS OF SCIENCE IN THE CENTURY. 



view or views of matter we may refer to the inves- 

 tigations of the French mathematician, Cauchy, as 

 to the motion of light in solid bodies and liquids. 

 He showed " that if matter were homogeneous, there 

 might be refraction, but there would be no dispersion. 

 All kinds of light would travel with the same velocity 

 in glass, just as they did in the air outside; and, 

 therefore, the mere fact that the different kinds of 

 light can be separated from one another in passing 

 through a prism, gives, at least, a hint that the mat- 

 ter of the prism is heterogeneous, is not infinitely 

 more fine-grained than the length of a wave of 

 any of the kinds of light which it enables us to sepa- 

 rate in their courses.'' * This kind of argument^ — 

 developed by Lord Kelvin — leads to the result that 

 400,000,000 in the inch is a rough approximation to 

 the heterogeneity or grained structure of matter. 



C. Other Methods of Estimating the Heterogeneity 

 of Matter. — In his Recent Advances in Physical 

 Science Prof. P. G. Tait gave an account of two 

 other methods ingeniously used by Lord Kelvin in 

 forming an estimate of the grained structure of mat- 

 ter. " The second method was founded upon con- 

 siderations of the amount of heat which would be 

 generated by electrical action between particles of 

 different materials when they were combined to- 

 gether. The third method was founded upon the 

 forces employed in drawing out a film of liquid, — 

 in fact (to take the simplest case), in blowing a soap- 

 bubble." The various methods yielded approxi- 

 mately the same result, " pointing consistently to 

 something not very largely differing from the 500,- 

 000,000th part of an inch as being the distance be- 

 tween the successive particles of matter in a liquid." 

 * P. G. Tait. Recent Advances, 1876, p. 304. 



