MOTION. 25 



iron and partly of brass, caused to emit a musical sound, or of 

 the same metal if its parts be not homogeneous, as a piece of 

 iron one portion of which is hard and crystallised, and the 

 other soft and fibrous ; the current resulting appears to be 

 due to the vibration, and not to heat engendered, as it ceases 

 immediately with the vibration. 



I venture therefore to think that in our present state of 

 knowledge, where the mutually impinging bodies are homo- 

 geneous, heat and not electricity is the result of friction and 

 percussion ; where the bodies impinging are heterogeneous, we 

 may safely state that electricity is always produced by friction 

 or percussion, although heat in a greater or less degree accom- 

 panies it ; but when we come to the question of the ratio in 

 which frictional electricity is produced, as determined by the 

 different characters of the substances employed, we find very 

 complex results. Bodies may differ in so many particulars 

 which influence more or less the development of electricity, 

 such as their chemical constitution, the state of their surfaces, 

 their state of aggregation, their transparency or opacity, their 

 power of conducting electricity, &c., that the normcs of their 

 action are very difficult of attainment. As a general rule, it 

 may be said that the development of electricity is greater 

 when the substances employed are broadly distinct in their 

 physical and chemical qualities, and more particularly in their 

 conducting powers ; but up to the present time the laws 

 governing such development have not been even approximately 

 determined. 



I have said, in reference to the various forces or affections 

 of matter, that either of them may, mediately or immediately, 

 produce the others ; and this is all I can venture to predicate 

 of them in the present state of science. Where at present 

 no immediate relation is established between any of them, 

 electricity generally forms the intervening link. 



Motion, then, will directly produce heat and electricity, and 

 electricity, being produced by it, will produce magnetism a 

 force which is always developed by electrical currents, at right 

 angles to the direction of those currents, as will be subse- 

 quently more fully explained. Light also is readily produced, 

 to all appearance, by motion, directly, when accompanying the 



