212 HELMHOLTZ 



other element, hydrogen, -which can be burnt just like carbon. 

 Hydrogen with carbon is a constituent of all combustible 

 vegetable substances, among others, it is also an essential 

 constituent of the gas which is used for lighting our streets 

 and rooms; in the free state it is also a gas, the lightest of 

 all, and burns when ignited with a feebly luminous blue 

 flame. In this combustion that is, in the chemical com- 

 bination of hydrogen with oxygen, a very considerable quan- 

 tity of heat is produced; for a given weight of hydrogen, 

 four times as much heat as in the combustion of the same 

 weight of carbon. The product of combustion is water, 

 which, therefore, is not of itself further combustible, for 

 the hydrogen in it is completely saturated with oxygen. 

 The force of affinity, therefore, of hydrogen for oxygen, 



FIG. 100 



like that of carbon for oxygen, performs work in combus- 

 tion, which appears in the form of heat. In the water which 

 has been formed during combustion, the force of affinity 

 is exerted between the elements as before, but its capacity 

 for work is lost. Hence the two elements must be again 

 .separated, their atoms torn apart, if new effects are to be 

 produced from them. 



This we can do by the aid of currents of electricity. In 

 the apparatus depicted in FIG. 100, we have two glass vessels 

 filled with acidulated water a and 01, which are separated in 

 the middle by a porous plate moistened with water. In both 



