* ELECTRIC POWER, 215 



temperature of the mercury was from 60°. to 80°. acquired no 

 electricity. The same was observed with glass rods $ these 

 came out, nevertheless, electric, when the temperature of the 

 mercury was no more thnn 40°. or 50°. c. -, but this electricity 

 is so weak and disproportionate to that which takes place when 

 the body is hoi, and tfie nercury cold, that I have always con- 

 sidered it with surprise. In order to conceive the cause of this "■ — owing to 

 ;.• • , _ ; ,,,,,, i -i the variation 



difference, it will be sufficient that I should observe, that a stick f tempera- 



of glass at 75°. c. requires only two minutes to cool down mre m the 

 , , . , , plunged body 



through 50°. c. in mercury, at 12°, c. whereas, when the same h e i n<r greater. 



stick at 12°. c. is plunged in mercury at 75°. c. it only causes a 



loss in the mercury of 4°. in the same period. 



V. To shew the influence of heat upon the electric power Similar expe- 



rini£iils ex** 

 still more strikingly, I plunged a thick cylinder of glass in tended. 



mercury at 80°. c It first became weakly electric, as 1 have 

 remarked, and afterwards non-excitable when its temperature 

 was the same as ihat of the mercury. But sometime afterwards, 

 and when the whole apparatus had advanced in cooling, I found 

 it extremely electrical, and the power afterwards gradually 

 became weaker accordingly, as by the progress of cooling, the 

 internal between the temperatures had become less. When 

 the mercury and the glass were entirely cooled, the electricity 

 was no longer observable. It is evident that the electricity 

 was here produced by the inequality of cooling which took 

 place between the two bodies from their inequality of con- 

 ducting power for heat. 



VI. I endeavoured to determine the nature of the electricity Natureorkind 

 of all these bodies plunged in mercury. Canton had asserted, city produced! 

 that glass comes out of mercury in the positive state. Van It depends on 

 Marum and Leroy found it negative. Ingenhousz found it S^on tite"" 1 

 positive by a slow immersion, and negative by a brisk one. weather. 

 After an attentive examination, I found that when the baro- 

 meter stands high, and the air is inclined to cold, then glass, 



amber, wax, paper, cotton, silk, and wool, are always negative, 

 whether the immersion be slow or brisk ; but that they are, on 

 the contrary, all positive, when the barometer is low, and the 

 air inclined to be warm. It is worthy of remark, that sulphur 

 is constantly positive at the same time when the other bodies 

 were the most strongly negative. I must also observe, that 

 during the whole summer, I found all the bodies positive in 



impure 



