THE ELECTRIC LIGHT. 663 
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. 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 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 
