Mr Canton found, that if A was the positive pole, and B the negative pole, when a tourmalin was excited by -heat, then when the tourmalin was not electrical, but be came so by cooling, the pole A was negative, and the pole B positive. Having placed a small tin cup of boil ing water on one end of his electrometer, which was supported by warm glass, while the pith balls were at the other end, he dropped a tourmalin into the water, and observed, that during the whole time of its being heated, and likewise during the operation of cooling, the balls were not at all electrified. This result seems, in some measure, to be contrary to the experiment of Mr Wilson, who covered all his tourmalins with grease, and when each of them was so warm as to preserve the grease liquid, he found their electrical property only a small degree weakened.
The following curious experiment was made by Mr Wilson. Having brought to a red heat one end of a glass tube, and exposed to it the negative pole of a tourmaline about three inches of the heated part of the glass was electrified negatively, an,l all the glass b. yowl it positively; and this property continued ce en after the glass was cold. The positive side of the toormalin was then applied to the same piece of heated glass; he found that the tube was electrified negatively alum a foot in length, without the least appearance of positive lee kity beyond it, and this negative electricity continued when the glass was nearly cold.
Dr Priestley made a number of curious experiments with the large totirmalin, which had been used by r Wilson and Mr Canton, and which had a convex and a flat side ; the convex side being always positive in cool ing, and the flat side negatise. In these experiments. Dr Priestley placed the tourmalin upon a pyrometer heated by a spirit lamp, in order to ascertain with acce racy whether the temperature was increasing, decreas• ing, or stationary. Dr Priestley began his experiments by laying the tourmalin on a plate of glass, he found that the glass had acquired an electricity equal and con trary to that of the side of the tourmaline which was in contact with it. A positively electrified feather, for ex ample, was always repelled at the distance of two inches by the flat side of the stone when its heat was increasing, while the glass attracted the feather ?t.the same or even at a greater distance. When the heat was drcre?sinc, the tourmalin attracted the feather, and the glass repell ed it at the distance of four or five inches. Wnen the
temperature, from increasing, began to decrease. the electricities were often reversed in less than a mmute, Dr Priestley now tried the effect of heating awl cool ing the tourmalin in contact with nonconductors and conductors, and hence he was led to a method of re• versing all the experiments that have been made upon the tourmalin, so that he could make the electricity of any pole just what he chose, by the application of proper substances.
Instead of the piece of glass already mentioned, De Priestley used a tourmalin, which he found to be affect ed exactly like the glasi.
He next imbedded the negative side of a tourmalin in hot scaling-wax, and when taken out of the wax it had positive, while the wax had negative electricity. The half of the tourmalin which was not in the wax was affect ed as if it had been in the open air, so that in cooling both sides were positive.
When Oa tourmalin was cooled in mercury contained in a china cup, it was always positive when taken out, while the mercury was left negative.
Having fastened the convex side of the large tourma lin to the end of a stick of scaling wax, he allowed it to cool, and then pressed the fiat side of it pretty bard against the palm of his hand, and found it to he strongly negative, contrary to what it would have been if exposed to the open air. The stone continued negative till it ac quired all the heat that it could receive from his hand, when its power decreased without changing the nathre of its electricity. The stone was then allowed to cool in the open air, and it became more strongly net,atile till it was quite cold. Hence the same side of the stone was negative, both in heating and cooling. Dr Priestley then heated the flat side by holding it near a red hot poker, and then touching it with the palm of his hand when it was intolerably hot, it became positive; allowing it to cool in the open air it became negative ; and by again tonchinect it with his hand, it again became positive. 11 this way he made the same side of the stone alternately positive and negative for a considerable time; and when the heat was such, that he could keep it in his hand, it acquired a strong positive eke tri, ity, which continued till it was brought to the heat or his hand. Analogous results were obtainA by fastening the wax to the flat side of the stone, and using the convex side.