CONDUCTING POWERS OF BODIES. 199 



quently, cohesive attraction is not very strong, is an im- 

 perfect conductor, and that copper, in which cohesion is 

 much more powerful, is a good conductor, we may be 

 disposed to consider that it is regulated by the closer ap- 

 proximation of the particles of matter. But in platinum 

 the corpuscular arrangement must be much more dense 

 than it is in copper, and yet it is, compared with it, a 

 very bad conductor.* 



We have now learnt that we may, by friction, excite 

 the electricity in a vitreous substance ; but it must not 

 be forgotten that we cannot increase the quantity whicli 

 is, under ordinary conditions, natural to the electric ; to 

 do so, we must in some way establish a channel of com- 

 munication with the earth, from which, through the 

 medium we excite, we draw our supply. We have the 

 means of confining this mighty force within certain limits 

 of quantity and of time. If we place bodies whicli are 

 susceptible of electrical excitation in a sensible degree 

 upon insulating ones, we may retain for a considerable 

 time the evidences of the excitement, in the same way as 

 ivith the Ley den jar; but there is a constant effort to 

 maintain a balance of conditions, and the body in whicli 

 we have accumulated any extraordinary quantity by con- 

 duction soon returns to its natural state. 



A very simple means may be adopted of showing what 

 is thought to be one of the many evidences in favour of 

 two electricities. If the wire carrying the current flow- 

 ing from the machine, is passed over paper covered with 

 nitrate of silver, it produces no change upon it ; but if 

 the wire which conveys the current to the instrument, 

 when it is excited, is passed over the same paper, the 

 silver salt is decomposed.f We may, however, explain 

 this result in a satisfactory manner, upon the hypothesis 



* Faraday's Speculation on the Nature of Matter, already 

 referred to. 



f Experimental Researches : by Dr. Faraday. Chemical Decom- 

 position, p. lot. 



