186 SCIENCE AND THE HUMAN MIND 



visible to the eye by watching coloured ions move 

 through an otherwise colourless liquid, and the 

 velocities calculated by theory have been thus 

 verified by experiment. 



The ionic theory, framed to explain the conduction 

 of electricity through liquids, has been adapted also 

 to the case of gases. But the development of this 

 subject belongs to a later period, and its consideration 

 must, for the present, be postponed. 



We have now reached what may be taken as the end 

 of the period in physical science most characteristic 

 of the nineteenth century. With the 

 unchanging, apparently eternal chemical 

 atoms, which " bore all the stamp of manufactured 

 articles," as basis, science had built up a scheme 

 which pictured a Universe of mechanism all but 

 understood, and actuated by a limited amount 

 of energy which could neither be increased nor 

 diminished. Even the atoms were waiting expect- 

 antly for someone to find a successful hypothesis 

 which would explain them away and turn them 

 into contortions of the all-embracing aether. It 

 seemed as though the main outlines of physical 

 knowledge had been laid down once for all, and 

 that the task of succeeding generations would 

 be but to fill in those outlines, and carry to yet 

 greater accuracy the measurements of known physical 

 constants. 



At a later stage of our enquiry we must trace the 

 revolution in outlook which soon disturbed this 

 prematurely completed scheme, and the immense 

 broadening and deepening of the range of known 



