662 FRAGMENTS OF SCIIWCK. 



counter and overcome resistance. .Flowing tlirougli 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 che 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, being 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. 



