662 FRAGMENTS OF SCTKNCK. 
counter 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 selfsame 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 incan- 
descent carbons. The space between them is filled with a 
blue flame which, bsing usually bent by the earth's mag- 
netism, receives the name of the Voltaic Arc.* 
- For seventy years, then, we have been in possession of 
this transcendent light without applying it to the illumi- 
nation of our streets and houses. Such applications sug- 
gested themselves at the outset, but there were grave 
* The part played by resistance is strikingly illustrated by the 
deportment of silver and thallium when mixed together and volatil- 
ized in the arc. The current first selects as its carrier the most 
volatile metal, which in this case is thallium. While 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 volatilize the 
silver. As the thallium disappears the current is forced to concen- 
trate its power; it presses the silver into its service, and finally fills 
the space between the carbons with a vapor 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. 
