444 FRAGMENTS OF SCIENCE 



warm the air. The zinc burns at the focus with a violet 

 flame, and we could readily determine the amount of heat 

 generated by its combustion. But zinc can be burned not 

 only in air, but in liquids. It is thus burned when acid- 

 ulated water is poured over it; it is also thus burned in 

 the voltaic battery. Here, however, to obtain the oxy- 

 gen 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 ab- 

 solutely invariable. Let our two batteries, then, continue 

 in action until an ounce of zinc in each of them is con- 

 sumed. In the one case the heat generated is purely do- 

 mestic, being liberated on the hearth where the fuel is 

 burned, 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 out- 



