442 FRAGMENTS OF SCIENCE 



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 plati- 

 num 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 re- 

 sistance 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 car- 

 bon 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 cur- 

 rent 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 Yoltaic 

 Arc. 1 



1 The part played by resistance is strikingly illustrated by the deportment 

 of silver and thallium when mixed together and volatilized 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 volatil- 

 ize 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 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 con- 



