360 PHILOSOPHICAL TRANSACTIONS. [aNNO 1 788. 



that the small plate will affect the electrometer very sensibly, and be quite suffici- 

 ent for the purpose ; whereas the large plate itself showed no clear signs of elec- 

 tricity. 



The 3d and last case is when the electricity to be ascertained is neither very 

 considerable in quantity, nor much condensed ; such is the electricity of the hair 

 of certain animals, of the surface of chocolate when cooling, &c. In this case 

 the best method is to apply a metal plate, furnished with an insulating handle, 

 like an electrophorus plate, to the electrified body, and to touch this plate with a 

 finger for a short time while standing in that situation ; which done, the plate is 

 removed, and is brought near an electrometer ; or its electricity may be com- 

 municated to the plate of a small condenser, as directed in the preceding case, 

 which will render the electricity more conspicuous. 



Having thus far described the surest methods of ascertaining the presence and 

 quality of electricity, when its quantity or degree of condensation is small, Mr. C. 

 now adds some further remarks on the subject of electricity in general, and which 

 have been principally suggested by what has been mentioned in the preceding 

 pages. From which he says it may be inferred, that the air, or in general any 

 substance, is a more or less perfect conductor of electricity, according as the 

 electricity which is to pass through it is more or less condensed ; so that if a 

 given quantity of electric fluid be communicated to a small brass ball, we may 

 take it away by simply touching the ball with a finger ; but if the same quantity 

 of electric fluid be communicated to a surface of about 100 or 1000 square feet, 

 the touching with the finger will hardly take away any part of it. If it be asked, 

 what power communicates the electricity, or originally disturbs the equilibrium 

 of the natural quantity of electric fluid in the various bodies of the universe ; we 

 may answer, that the fluctuating electric state of the air, the passage of electrified 

 clouds, the evaporation and condensation of fluids, and the friction arising from 

 divers causes, are perpetually acting on the electric fluid of all bodies, so as eitTier 

 to increase or diminish it, and that to a more considerable degree than is gene- 

 rally imagined. 



Mr. C. lastly concludes with briefly proposing an explanation of the production 

 of electricity by friction, which is dependent on the above stated proposition, viz. 

 that bodies are always electrified in some degree ; and also on the well known 

 principle of the capacity of bodies for holding electric fluid being increased by the 

 proximity of other bodies in certain circumstances. It seems to me, he says, 

 that the cylinder of an electrical machine must always retain some electricity of 

 the positive kind, though not equally dense in every part of its surface ; there- 

 fore, when the part of it a is set contiguous to the rubber, it must induce a 

 negative electricity in the rubber. Now, when by turning the cylinder, another 

 part of it B, which suppose to have a less quantity of positive electricity than the 



