98 CORRELATION OF PHYSICAL FORCES. 



transmitting light, is most unfavourable to its transmission of 

 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 



