90 EFFECT OF THE PHYSICAL FORCES PART i. 



pulsations which pass along the wire of the former, and 

 through the battery of the latter, impulses probably 

 generated by the action of the discharge along the 

 wires ? ' 



The action of magnetism and electricity on light is 

 similarly illustrated by the rotation of the plane of 

 polarization. Sir John Herschel was the first who tried 

 to rotate the plane of polarization of a ray of light by 

 surrounding it with a spiral wire electrized by the great 

 battery of two enormous plates of copper and zinc at 

 the London Institution, but he obtained no evidence of 

 any such action. Long afterward Professor Faraday 

 succeeded by sending a ray of light through a piece of 

 silico-borate of lead, which formed the core of a mag- 

 netic helix. The silico-borate took on a quasi-crystal- 

 lised state during the passage of the electric current 

 round it, giving it for the moment the property of 

 circular polarization, analogous to that of glass in a 

 state of tension or compression. 



Substances vary exceedingly in the facility with which 

 they transmit electricity ; even the same substance under 

 another form differs remarkably in that property : char- 

 coal, which next to the metals is the best conductor 

 known, when under the form of diamond is quite 

 impervious to electricity. In general, substances that 

 are the best conductors of heat are also the best con- 

 ductors of electricity, as for example the metals, which 

 however, possess the transmissive property in very dif- 

 ferent degrees. Silver and copper are the best con- 

 ductors, lead one of the worst ; its resistance to the 

 passage of electricity is twelve times greater than that 

 of silver and copper, consequently it becomes twelve 

 times as hot, for when a current of electricity is 

 impeded it is changed into heat. So great is the 

 resistance offered by a fine platinum wire, that the 

 heat amounts to 3280 and the wire is melted, a striking 



