THE PROGRESS OF PHYSICS. 165 



of the wave-length ; and the same interpretation may 

 account for the absence of conclusive evidence of 

 polarisation. 



Summary. — Of what is meant by an electric 

 charge, the nineteenth century has left us ignorant, 

 hut many laws of electrical phenomena have been 

 discovered, and that electrical radiations are best 

 interpreted in terms of ethereal waves is generally 

 conceded. Indeed it has become a question whether 

 all matter may not be resolvable into aggregates of 

 electric charges of opposite sign. But both as regards 

 theory and as regards practical applications, astound- 

 ing as the progress of these has been,^ the twentieth 

 century is pregnant with possibilities of development. 



THEORIES OF MATTER. 



Very early in the history of science the idea arose 

 in the minds of enquirers that matter might consist 

 of an aggregation of invisible particles separated by 

 interspaces. This became a precise scientific hypo- 

 thesis about a century ago, when Dalton developed 

 his Atomic Theory. During the nineteenth century 

 the hypothesis has been in several ways developed 

 as fresh facts came to light. 



When we see water becoming vapour and again be- 

 coming ice, when we see what is usually a gas lique- 

 fied and even solidified, when we watch the crystal 

 of sugar melting away in the teaspoon or a crystal 

 of alum growing in a solution of alum, when we con- 

 sider that many bodies, like iron, expand when heated 

 and contract again as they cool, when we observe that 

 a gas may diffuse through another or even through a 



* A fascinating exposition of modern views will be found 

 in an article by Prof. Oliver Lodge, International Monthly 

 I. (1900), pp. 483-530. 



