lame; while fragments of
diamond and plumbago rapidly disappeared as if reduced to vapour.
[Footnote: In the concluding lecture at the Royal Institution in June,
1810, Davy showed the action of this battery. He then fused iridium,
the alloy of iridium and osmium, and other refractory substances.
'Philosophical Magazine,' vol. xxxv. p. 463. Quetelet assigns the
first production of the spark between coal-points to Curtet in 1802.
Davy certainly in that year showed the carbon light with a battery of
150 pairs of plates in the theatre of the Royal Institution ('Jour.
Roy. Inst.' vol. i. p. 166).]

The first condition to be fulfilled in the development of heat and
light by the electric current is that it shall encounter and overcome
resistance. Flowing through a perfect conductor, no matter what the
strength of the current might be, neither heat nor light could be
developed. A rod of unresisting copper carries away uninjured and
unwarmed an atmospheric discharge competent to shiver to splinters a
resisting oak. I send the self-same current through a wire composed
of alternate lengths of silver and platinum. The silver offers little
resistance, the platinum offers much. The consequence is that the
platinum is raised to a white heat, while the silver is not visibly
warmed. The same holds good with regard to the carbon terminals
employed for the production of the electric light. The interval
between them offers a powerful resistance to the passage of the
current, and it is by the gathering up of the force necessary to burst
across this interval that the voltaic current is able to throw the
carbon into that state of violent intestine commotion which we call
heat, and to which its effulgence is due. The smallest interval of
air usually suffices to stop the current. But when the carbon points
are first brought together and then separated, there occurs between
them a discharge of incandescent matter which carries, or may carry,
the current over a considerable space. The light comes almost wholly
from the incandescent carbons. The space between them is filled with
a blue flame which, being usually bent by the earth's magnetism,
receives the name of the Voltaic Arc. [Footnote: The part played by
resistance is strikingly illustrated by the deportment of silver and
thallium when mixed together and volatilised in the arc. The current
first selects as its carrier the most volatile metal, which in this
case is thallium. Wh