1879.] on the Electric Lhjht. 3 



For seventy years, then, wc have been in possession of this trans- 

 cendent liglit without applying it to the illumination of our streets 

 and liouses. Sueh 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 first of them, I believe, 

 by Staite, and the most successful by Duboscq, Foucault, and Serrin, 

 who have been succeeded by a multitude of other inventors, to some 

 of whom I shall subsequently refer. By such arrangements the first 

 difficulty was practically overcome ; but the second is a graver one, 

 being 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. We could boil water 

 by electricity, but it would not be an economical way of supplying 

 our baths and washhouses. These considerations are now to bo 

 applied. The metal zinc can be burnt like j)aper ; it might be 

 ignited in a flame, but I will avoid the introduction of all foreign 

 heat and 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 burnt not only in air but in liquids. 

 It is thus burnt when acidulated water is poured over it ; it is also 

 thus burnt 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 hydi'ogen. 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 manu- 

 facture our electric light. 



Beford 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 i^latinum glows with a white heat, while the copj)er 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 



B 2 



