LXXI. A letter from John Canton, M. A. and F. R. S. to Benjamin Franklin, LL. D. and F. R. S. containing some remarks on Mr. Delaval's electrical experiments [ 457 ] L X X I. A L e tte r fro m John Canton, M . A . an d F. R . S. to Benjamin Franklin, L L . D . and F. R . S. containing fome Re­ marks on M r . DelavalV EleElrical E xperi­ ments. D ear Sir, Read Feb. 4, 1762. T i y T R . Delaval, in his curious eleCtrica) 1 V 1 experiments, found that Portland ftone, com mon tobacco-pipe, &c. would readily con­ duct the eleCtrical fluid, w hen very hot, or when quite co ld ; but were non-conductors in an intermediate ftate. As no one, that I know of, has yet attempted to account for this, I fhall fubmit the following fo- lution to your judgm ent. T h e ftone, tobacco-pipe, wood, See. I apprehend, conduCt w hen cold, by the moifture they contain in that ftate; w hen their moifture is evaporated by heat, they become non-conduCtors 3 and w hen they are m ade very hot, the hot air at, or near their furfaces, will conduCt, and the bodies appear to be conductor# again. T o prove that hot air will conduCt the eleCtricai fluid, let the end o f a poker, w hen red-hot, be brought, but for a m om ent, w ithin three or four inches o f a fmali electrified body, and its eleCtrical power will be almoft, if not entirely deftroyed. A nd if excited amber, &c. be held within an inch of the flame of a candle, it will lofe its elec- N n n 2 tricity D ow nl oa de d fr om h tt ps :/ /r oy al so ci et yp ub li sh in g. or g/ o n 05 A pr il 2 02 1 (ricity before it has acquired a fenfibie degree of heat (i ). T h at gl&fs is a condudor in damp weather, on ac- • count of the moiflure on its furface, is well known ; as alfo, that warming it a little will render it a non- condudor ; and that a great degree o f heat will make it feem to be a condudor again. Now tobacco-pipe, wood, &c. will not only attrad the moifture o f the air to their furfaces, but will alfo abforb it; whence they are condudors in dry w eather; and require more heat than glafs, as well as a longer continuance in it, to render them non-condudors. It is remark­ able, that tobacco-pipe, after it begins to cool, will become a condudor again, fooner than mod: other fubftances, and much fooner than wood. T h e caufe of this appears to me, to be the tobacco-pipe’s ab- forbing the moidure of the air fader than m od other [ 458 ] (1) I have obferved alfo, that the Tourmalin, Brazil T opaz, and Brazil Emerald, will give much flronger figns of eledricity while cooling, after they have been held'about a minute within two inches of an almoft furrounding fire, where the air is a con­ ductor, than they ever will after heating them in boiling water. And if both fides of either of thofe {tones be equally heated, but in a lefs degree than will make the furrounding air a condudor, the eledricity of each fide, whether plus or minus, will continue fo, all the time the {tone is both heating and cooling; but will increafe while i t is heating, and decreafe while it is cooling. Whereas, if the heat be {efficient to make the furrounding air condud the eledric fluid from the pofitive fide of the ftoile to the negative fide or it, while heating; the eledricity of each fide will incjeafe, while the ftone is cooling, and be contrary to what it was, while the ftone was heating. See the Piiilofophical Tranfadions,. Vol. L I. p. 403 and 404. fubftances* D ow nl oa de d fr om h tt ps :/ /r oy al so ci et yp ub li sh in g. or g/ o n 05 A pr il 2 02 1 [' 459 ] fuhftances, and much fafter than w o o d : for the fur- faces of tobacco-pipe and wood being wetted, the furface o f the wood will continue wet m uch longer than the furface of the tobacco-pipe. T h a t tobacco-pipe does not become a non-con- du&on by a particular degree o f heat, w ithout eva­ porating its moiflure, is evident, from the following experiments. Jf three or four inches ol one end of a tobacco-pipe, o f more that a foot in length, be made red-hot, without fenfibly heating the other e n d ; this pipe will prove a ready condudor, through the hot air furrounding one part of it, and the moifture con­ tained in the o th e r; although fome part o f it muff have the degree o f heat o f a non-condudlor. But if the whole pipe be made red-hot, and fuffered to cool, till it has only fuperficial moiflure enough to make it a good co nductor; and then three or four inches of one end be again made red-hot, it will become a non­ conductor. A nd if a nail be placed at, or near each end o f a longifh folid piece of any of the abforbent bodies, above-mentioned, fo that the point of each nail may he about half the thicknefs of the body, w ithin its furface:; this body, by heat, may be made a non-conduCtor externally, or fuperficially, while it remains a good conductor internally: for the eledric fluid will pafs readily from one nail to the other, through the middle o f the body, when it will not pafs on its furface; and even when the internal parts of the body are in an equal degree of heat with the external; as they mufl foon be, after it begins to cool. But if the fame body be expofed, for a fhort time,, to a greater degree of heat than before; or if it be kept. D ow nl oa de d fr om h tt ps :/ /r oy al so ci et yp ub li sh in g. or g/ o n 05 A pr il 2 02 1 [ 4«o ] kept longer in the fame heat, it will become a non­ conductor entirely. In making the above experiments, I ufed the little electrometer, which I have deferibed in the forty- eighth volume of the Philofophical Tranfa&ions, p. 783, and fupported it by fealing-wax, or warm glafs. I well remember your acquainting me, that M r. Delavai did not approve of the above manner o f ac­ counting for his experiment on tobacco-pipe, foon after you related it to him, which was fome time laft fu m m er: but as it ftili appears fatisfaCtory to me, notwithftanding what that gentleman has lately offered againft it (2), your laying it before the Royal Society will oblige, Dear Sir, Your moft obedient and moft humble fervant, Spital-Squarc, _ - * January 21, 1762. J o h n C a n t o n . ( 2 ) See a Letter from M r. Delavai to Mr. W ilfo n , in the firft part o f the fifty-fecond volume o f the Philofophical T ra n s­ actions. P. s. D ow nl oa de d fr om h tt ps :/ /r oy al so ci et yp ub li sh in g. or g/ o n 05 A pr il 2 02 1 [ +