264 CONNECTION OF ELECTRICITY AND MAGNETISM. 



in this country one grain weight on eight sqnare feet. In some of Dr. 

 Joule's experiments, the medium has exerted a tension of 200 pounds 

 "weig'ht per square inch. 



But the medium, in virtue of the very same elasticity by which it is 

 able to transmit the undulations of light, is also able to act as a spring. 

 When properly wound up, it exerts a tension different from the mag- 

 netic tension, by which it draws oppositely electrified bodies together, 

 produces effects through the length of telegraphic wires, and, when of 

 sufficient intensity, leads to the rupture and explosion called lightning. 



These are some of the already" discovered properties of that which has 

 often been called vacuum, or nothing at all. They enable us to resolve 

 several kinds of action at a distance into actions between contiguous 

 parts of a continuous substance. Whether this resolution is of the 

 nature of explication or complication, I must leave to the metaphysicians. 



