THE ELECTRIC LIGHT. 555 



the most successful by Duboscq, Foucault, and Serrin, who have been 

 succeeded by Holmes, Siemens, Browning, Carre, Gramme, Lontin, and 

 others. By such arrangements the first difficulty was practically over- 

 come ; but the second, a graver one, is probably inseparable from the 

 construction of the voltaic battery. It arises from the operation of 

 that inexorable law which, throughout the material universe, demands 

 an eye for an eye and a tooth for a tooth, refusing to yield the faintest 

 glow of heat or glimmer of light without the expenditure of an abso- 

 lutely equal quantity of some other power. Hence, in practice, the 

 desirability of any transformation must depend upon the value of the 

 product in relation to that of the power expended. The metal zinc 

 can be burned like paper ; it might be ignited in a flame, but it is pos- 

 sible to avoid the introduction of all foreign heat and to burn the zinc 

 in air of the temperature of this room. This is done by placing zinc- 

 foil at the focus of a concave mirror, which concentrates to a point the 

 divergent electric beam, but which does not 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 acidulated 

 water is poured over it ; it is also thus burned in the voltaic battery. 

 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 is 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, con- 

 tinue 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 burned, that is to say in the cells of the bat- 

 tery itself. In the other case, tlie heat is in part domestic and in part 

 foreign — in part within the battery and in part outside. One of the 

 fundamental truths to be borne in mind is that the sum of the foreign 

 and domestic — of the external and internal — heats is fixed and invaria- 

 ble. Hence, to have heat outside you must draw upon the heat w^ithin. 

 These remarks apply to the electric light. By the intermediation of the 



