TIIK KLKCTllIC LIGHT. G63 



difficulties in their way. The first difficulty arose from the 

 waste of the carbons, which are dissipated in part by 

 ordinary combustion, and in part by the electric transfer 

 of matter from the one carbon to the other. To keep the 

 carbons at the proper distance asunder regulators were 

 devised, the earliest, I believe, by Staite, and 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 overcome; 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 absolutely 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 possible 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 concen- 

 trates 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 dis- 

 lodge 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. Fullv 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 



