ile it continues abundant, the passage of the
current is so free--the resistance to it is so small--that the heat
generated is incompetent to volatilise the silver. As the thallium
disappears the current is forced to concentrate its power; it presses
the silver into its service, and finally fills the space between the
carbons with a vapour which, as long as the necessary resistance is
absent, it is incompetent to produce. I have on a former occasion
drawn attention to a danger which besets the spectroscopist when
operating upon a mixture of constituents 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.]

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 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, 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 burnt 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 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 readil