POSITIVE AND NEGATIVE. 



203 



and condition ; but we may accept as a general axiom, but by no means as 

 a law, that when two bodies are rubbed together, that which gets the hotter 

 in the process takes the negative kind of electricity. In the following list 

 the substances have been so arranged that each is negatively electrified 



Fig. 205. Positive and negative. 



by those preceding, and positively by those succeeding it. I, cat's skin; 

 2, glass ; 3, woollen stuffs ; 4, feathers ; 5, wood ; 6, paper ; 7, silk ; 

 8, shellac ; 9, rough glass. We append a list of conducting and non- 

 conducting bodies in their order : 



Metals. 



Lime, coal, or coke. 



Saline mixtures. 



Ice. 



India-rubber. 



Marble. 



Porcelain. 



Resin. 



CONDUCTORS. 

 Pure water. 

 Vegetable tissues. 

 Animal tissues. 



NON-CONDUCTORS. 

 Dry gases. 

 Paper. 

 Wool. 



Silk. 

 Shell-lac. 



Hot aii. 

 Steam. 

 Rarefied air. 



Diamond. 

 Glass. 

 Wax. 

 Sulphur. 



Fig. 206. The Battery. 



It should be observed that the degree of value as a conductor or non- 

 conductor depends somewhat upon the atmosphere. For instance, glass is 

 an excellent insulator, or non-conductor, when dry, but when wet it changes 

 to a conductor. So insulators are at times covered with a solution of shell-lac, 

 or fat, to keep away moisture. We may reasonably conclude that bodies 



