THE ELECTRIC LIGHT. 423 



poured over it; it is also thus burnt in the voltaic bat- 

 tery. Here, however, to obtain the oxygen necessary 

 for its combustion, the zinc has to dislodge the hydrogen 

 with which the oxygen is combined. The consequence 

 is that the heat due to the combustion of the metal in 

 the liquid falls short of that developed by its combustion 

 in air, by the exact quantity necessary to separate the 

 oxygen from the hydrogen. Fully four-fifths of the 

 total heat are used up in this molecular work, only one- 

 fifth remaining to warm the battery. It is upon this 

 residue that we must now fix our attention, for it is 

 solely out of it that we manufacture our electric light. 

 Before you are two small voltaic batteries of ten 

 cells each. The two ends of one of them are united by 

 a thick copper wire, while into the circuit of the other 

 a thin platinum wire is introduced. The platinum 

 glows with a white heat, while the copper wire is not 

 sensibly warmed. Now an ounce of zinc, like an ounce 

 of coal, produces by its complete combustion in air a 

 constant quantity of heat. The total heat developed 

 by an ounce of zinc through its union with oxygen in 

 the battery is also absolutely invariable. Let our two 

 batteries, then, continue in action until an ounce of 

 zinc in each of them is consumed. In the one case the 

 heat generated is purely domestic, being liberated on 

 the hearth where the fuel is burnt, that is to say in the 

 cells of the battery itself. In the other case, the heat 

 is in part domestic and in part foreign in part within 

 the battery and in part outside. One of the funda- 

 mental truths to be borne in mind is that the sum of 

 the foreign and domestic of the external and internal 

 heats is fixed and invariable. Hence, to have heat 

 outside, you must draw upon the heat within. These 

 remarks apply to the electric light. By the interme- 

 diation of the electric current the moderate warmth 



