INTRODUCTION 3 



atom. In both these applications it has been 

 very successful, but it involves the abandonment 

 in problems of radiation and atomic structure of 

 hitherto accepted dynamical principles. 



Again, the theory of relativity, founded by 

 Einstein and Minkowski, leads to another revolu- 

 tion in scientific thought. The ideas of absolute 

 space and absolute time are held by this new 

 view to be unwarranted though perhaps natural 

 figments of the imagination. If we keep to the 

 only space and time we really know, they can 

 be but space and time as recorded by some ob- 

 server ; they will not, it appears, be the same for 

 all observers ; they are relative and not absolute. 

 The real unity is a complex of both, and this space- 

 time is absolute and not relative. Strange to 

 say, this theory, as followed by Einstein, has led 

 to a new outlook on the agelong problem of 

 gravitation. The motion of the planets, the 

 weight of a stone, may, it seems, be due to 

 something we must represent as a curvature of 

 space-time rather than to the now familiar but ever 

 mysterious gravitational attraction of Newton. 



As we shall see in the following pages, the 

 chief work of modern experimental physicists is 

 undertaken and interpreted by the aid of atomic 

 and molecular conceptions. The theory of the 

 conduction of electricity through liquids, based 

 originally on the work of Faraday, and slowly 

 matured by HIttorf, Kohlrausch, Arrhenius, and 

 many others, had accustomed our minds to the 

 conception of electric conduction by means of the 

 motion of charged particles, called by Faraday 

 "ions" — the travellers. Each ion consists of an 



