THE ELECTRIC LIGHT 443 



For seventy years, then, we have been in possession 

 of this transce-ndent light without applying it to the illu- 

 mination of our streets and houses. Such applications 

 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 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, Carr^, 

 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 uni- 

 verse 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 quan- 

 tity 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 burned like paper; it might be 

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

 tion 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 



stituents volatile in different degrees. When, in 1872, I first observed the effect 

 here described, had I not known that silver was present, I should have inferred 

 its absence. 



