118 PROGRESS OF SCIENCE IN THE CENTURY. 



change we do not know, though hypothetical sugges- 

 tions have been offered. 



Photochemistry or the study of the effects of 

 radiant energy (light) on chemical processes is still 

 incipient; though its results have led to the develop- 

 ment of photography, the influence of light on the 

 green leaf remains an unread riddle. 



Electrochemistry. — It is a familiar fact that if a 

 rod of zinc and a rod of platinum he immersed in 

 dilute sulphuric acid (which does not attack either of 

 them separately), and if the ends of the two rods 

 projecting out of the liquid be apposed or connected 

 by a metal wire, the zinc is dissolved, the hydrogen 

 of the sulphuric acid accumulates on the platinum, 

 and there has come into existence an electric current 

 — a form of energy — which can be made to do work. 

 The source of this energy is in the chemical process, 

 in the heat evolved by the solution of the zinc. By 

 using heat as the common standard of measurement, 

 we are able to prove that a certain amount of poten- 

 tial chemical energy available at the outset is exactly 

 equivalent to the amount of electrical energy pro- 

 duced plus the heat evolved at the seat of the reaction. 



From the study of comparatively simple experi- 

 ments like that above referred to, always in the light 

 of the doctrine of the conservation of energy, electro- 

 chemistry has evolved into an important and elabo- 

 rate department of science. 



Faraday distinguished bodies, e.g., metals, which 

 conduct electrical currents without suffering any 

 material change beyond that of heating, from other 

 bodies, such as salts and aqueous solutions of acids 

 and bases, in which the conducted current induces 

 chemical change. " In such conductors of the second 

 class, or electrolytes, the movement of electricity 



