98 COEEELATION OF PHYSICAL FOECES. 



transmitting light, is most unfavourable to its transmission oi 

 electricity, transparent solids being very imperfect conductors 

 of electricity ; so all gases readily transmit light, but are 

 amongst the worst conductors of electricity, if, indeed, prop 

 erly speaking, they can be said to conduct at all. 



The conduction of electricity by different classes of bodies 

 has been generally regarded as a question of degree : thus 

 metals were viewed as perfect conductors, charcoal less so, 

 water and other liquids as imperfect conductors, &c. But, 

 in fact, though between one metal and another the mode of 

 transmission may be the same and the difference one of de 

 gree, a different molecular effect obtains, when we contrast 

 metals with electrolytic liquids and these with gases. 



Attenuated gases may be, in one sense, regarded as non 

 conductors, in another, as conductors ; thus if gold-leaves be 

 made to diverge, by electrical repulsion, in air at ordinary 

 pressure, they in a short time collapse ; while in highly-rare 

 fied air, or what is commonly termed a vacuum, they remain 

 divergent for days ; and yet electricity of a certain degree of 

 tension passes readily across attenuated air, and with diffi 

 culty across air of ordinary density. 



Again, where the electrical terminals are brought to a 

 state of visible ignition, there are symptoms of the transmis 

 sion of electricity of low tension across gases ; but no such 

 effects have been detected at lower temperatures. All this 

 presents a strong argument in favour of the transmission of 

 electricity across gases being effected by the disruptive dis 

 charge, and not by a conduction similar to that which takes 

 place with metals or with electrolytes. 



The ordinary attractions and repulsions of electrified 

 bodies present no more difficulty when regarded as being pro 

 duced by a change in the state or relations of the matter af 

 fected, than do the attractions of the earth by the sun, or of 

 a leaden ball by the earth ; the hypothesis of a fluid is not 

 considered- necessary for the latter, and need not be so for the 



