422 FKAGMENTS OP SCIENCE. 



For seventy years, then, we have been in possession 

 of this transcendent light without applying it to the 

 illumination of our streets and houses. Such applica- 

 tions suggested themselves at the outset, but there were 

 grave difficulties in their way. The first difficulty 

 arose from the waste of the carbons, which are dissi- 

 pated 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, Sie- 

 mens, Browning, Carre, Gramme, Lontin, and others. 

 By such arrangements the first difficulty was practi- 

 cally overcome; but the second, a graver one, is pro- 

 bably 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 desira- 

 bility of any transformation must depend upon the value 

 of the product in relation to that of the power expended. 

 The metal zinc can be burnt like paper; it might be 

 ignited in a flame, but it is possible to avoid the intro- 

 duction of all foreign heat and to burn the zinc in air 

 of the temperature of this room. This is done by plac- 

 ing 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 de- 

 termine the amount of heat generated by its combus- 

 tion. But zinc can be burnt not only in air but in 

 liquids. It is thus burnt when acidulated water is 



