010 PHILOSOPHICAL TRANSCATIONS. [aNNO I762. 



That glass is a conductor in clamp weather, on account of the moisture on its 

 surface, is well known ; as also that warming it a little will render it a non-con- 

 ductor ; and that a great degree of heat will make it seem to be a conductor 

 again. Now tobacco-pipe, wood, &c. will not only attract the moisture of the 

 air to their surfaces, but will also absorb it ; whence they are conductors in dry wea- 

 ther ; and require more heat than glass, as well as a longer continuance in it, to 

 render them non-conductors. It is remarkable that tobacco-pipe after it begins 

 to cool, will become a conductor again sooner than most other substances and 

 much sooner than wood. The cause of this appears to me to be the tobacco- 

 pipe's absorbing the moisture of the air faster than most other substances, and 

 much faster than wood ; for the surfaces of tobacco-pipe and wood being wetted, 

 the surface of the wood will continue wet much longer than the surface of the 

 tobacco-pipe. 



That tobacco-pipe does not become a non-conductor by a particular degree of 

 heat without evaporating its moisture is evident, from the following experiments. 

 If 3 or 4 inches of one end of a tobacco-pipe, of more than a foot in length, be 

 made red-hot, without sensibly heating the other end, this pipe will prove a ready 

 conductor, through the hot air surrounding one part of it, and the moisture con- 

 tained in the other ; though some part of it must have the degree of heat of a 

 non- conductor. But if the whole pipe be made red-hot, and suffered to cool till 

 it has only superficial moisture enough to make it a good conductor, and then 3 

 or 4 inches of one end be again made red-hot, it will become a non-conductor. 

 And if a nail be placed at or near each end of a longish solid piece of any of the 

 absorbent bodies above- mentioned, so that the point of each nail may be about 

 half the thickness of the body within its surface ; this body by heat may be made 

 a non-conductor externally or superficially, while it remains a good conductor in- 

 ternally : for the electric fluid will pass readily from one nail to the other, through 

 the middle of the body, when it will not pass on its surface ; and even when the 

 internal parts of the body are in an equal degree of heat with the external ; as 

 they must soon be after it begins to cool. But if the same body be exposed for 

 a short time, to a greater degree of heat than before , or if it be kept longer in 

 the same heat, it will become a non-conductor entirely. 



P. S. Having formerly observed that the friction between mercury and glass in 

 vacuo, would not only produce the light of electricity, as in the luminous baro- 



will make the surrounding air a conductor, the electricity of each side, whether plus or minus, will 

 continue so, all the time the stone is both heating and cooling, but will increase while it is heating, 

 and decrease while it is cooling. Whereas, if the beat be sufficient to make the surrounding air con- 

 duct the electric fluid from the positive side of the stone to the negative side of it, while heating j 

 the electricity of each side will increase, while the stone is cooling, and be contrary to what it was, 

 whilethe stone was heating. See the Phil. Trans, vol. li. p. 403 and 40+. — Orig. 



