2\6 ELECTRIC POWER. 



impure mercury, or such as was alloyed with tin, and negative 



at the same time injure mercury. 



Other experi- I ma de several experiments to ascertain the influence of 

 ments on teni- - . . ... r , . , 



perature. temperature upon positive or negative electricity ; one' of which 



I shall mention. On the 10th of July, the wind being N. E. and 

 clear, temperature 21°. c. j at 1 lh. in the morning, my heated 

 glass rod came out of mercury in the negative state. I raised 

 the temperature of the mercury to 100\ my rod was then 

 without electricity, while the mercury imparted heat to it ; but 

 as soon as the whole began to cool, the rod became strongly 

 negative in all Its immersed parts. Some time afterwards, and 

 constantly leaving it in the mercury, I found it positive at its 

 extremity, and negative in all the rest. It must be remarked, 

 that the vessel containing the mercury was conical, and the 

 lower part being thin, the cooling was more rapid here than 

 elsewhere. When the mercury was no higher here than 34°. c. 

 the rod came out without electricity, but it continued to be so 

 in the mercury at 26\ c. and constantly negative. Having then 

 heated the rod a little above the mercury, at 34*. I withdrew 

 it positive from the mercury ; but it was always negative in 

 mercury at 26°. 



It is, therefore, established, that at equality of temperature, 

 the rod is not excitable in mercury j though it is positive when it 

 is only a little warmer than that fluid ; and negative, when 

 there is a great interval between the temperatures. 



I must remark, that these different degrees of temperature 

 do not change the electric state where the air tends to cold, 

 or the barometer is low j for in the first case the rod is always 

 negative, and in the second always positive. 

 Friction upon VII. These influences of the temperature upon the electric 

 w0 ° * power, are not peculiar to mercury j they are observable like- 



wise, in the friction of the same substances upon wool. In 

 fact, if they be cooled in mercury at 1 2°. their electric power 

 disappears equally in friction. It is not excited in amber until 

 the sixth double friction, in wax at the eighth, in glass at the 

 ninth, and in sulphur at the tenth. In this circumstance, if, 

 after having exacted the electric power of glass, it be left at 

 repose for 30 seconds, it is remarked that it becomes again 

 non-excitable, and it will require four double frictions to re- 

 animate it. It continues afterwards to become electric at each 



friction, 



