ELECTRICITY. 



ncHh|>it>r niity t', it will avoid going on to touch that extremity 

 , inclining towards the point a ; and all the points 

 "*"*""" ' situated between a and the extremity H. will art 

 ii|nni it liy attraction ; to that we shall not have precise- 

 ly the inverse of the pnadfag effects, because, in both 

 cue*, the thread is attracted by the middle part of the 

 tourmalin." 



1 1' a tourmalin, when in a state of excitation by heat, 

 is broken into two p-irt-, however small, each fragment 

 ha* two opposite pole-, a phenomenon analogous to 

 what takes place in a broken magnet. Mr Canton cut 

 a large irregular tourmalin into three pieces, anira piece 

 being cut from the positive, and another from the ne- 

 gative pole, the outer side of the piece which lie cut 

 from the negative pole was negative when cooling, and 

 the outer side of the piece which he cut from the po-i- 

 tive pole was positive while cooling, the opposite enil* 

 of all the pieces possessing a contrary electricity. The 

 middle piece had the same properties a* in the outer tour- 

 malins, the positive end remaining positive, and the ne- 

 gative end negative. He obtained the same results from 

 two other tourmalins that had been cut out of a large 

 one. Dr Priestley, who broke a tourmalin by acci- 

 dent into three fragments, one of which was 96 grains, 

 another 10 grains, and another one grain, found that 

 the largest fragment was not injured by the accident, 

 and its two poles were unaltered. 



Mr Canton found, that if A was the positive pole, 

 and B the negative pole, when a tourmalin was ex- 

 cited by heat, then when the tourmalin was not elec- 

 trical, but became so by cooling, the pole A was nega- 

 tive, and the pole B positive. Having placed a small 

 tin cup of boiling 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 in- 

 to 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 ex- 

 periment 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 pro- 

 perty only a small degree weakened. 



The following cunous 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 tour- 

 malin, about three inches of the heated part of the 

 glass was electrified negatively, and all the glass be- 

 yond it positively ; and this property continued even 

 after the glass was cold. The positive side of the tour- 

 malin was then applied to the same piece of heated 

 glass ; he found that the tube was electrified negative- 

 ly about a foot in length, without the least appearance 

 of positive electricity beyond it, and this negative elec- 

 tricity continued when the glass was nearly cold. 

 Dr Priew- 1 )r Priestley made a number of curious experiments 

 ley'i erpt- with the large tourmalin, which had been used by Mr 



_:__*- __ * 



on Wilson and Mr Canton, and which had a convex and 

 a flat side ; the convex side being always positive in 

 cooling, and the flat side negative. In these experi- 

 ments, Dr Priestley placed the tourmalin upon a py- 

 rometer heated by a spirit lamp, in order to ascertain 

 with accuracy whether the temperature was increasing, 

 decreasing, or stationary. Dr Priestley began his ex- 

 periments by laying the tourmalin on a plate of glass, 

 and he found that the glass had acquired an electricity 

 equal and contrary to that of the side of the tourmalin, 

 which was in contact with it A positively electrified 



r, for example, was always repelled at the di - 

 of two inches by the flat Mile of the Mnnr when K!l 

 its heat was increasing, while tin- .1 the """" JT'^^ 



feather at the *ame or even at a greater di>t.ince. When J^'s "m. 

 the heat was dtcrrasiii", the tourmalin attracted the rto 

 feather, and the trla^-. repelled it at the dUtaiuv of four tlit louinu- 

 or five inches When the temperature, from incrc.--.-u lin - 

 ing, began to decrease, the electricities were often re- 

 versed in le-- than a minute. 



Dr Priestley now tried the effect of heating and 

 cooling the tourmali.i in contact with non-conductors 

 and conductors, and hence he was led to a method of 

 reversing all the experiments that have been made iijxm 

 the tourmalin, so that he could make the electricity 

 of any pole just what he chose, by the application of 

 proper substance*. 



Instead of tlie piece of glass already mentioned, Dr 

 Priestley used a tourmalin, which he found to be affect- 

 ed exactly like the glass. 



He next imbedded the negative side of a tourmalin 

 in hot sealing-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 affected as if it had been in the open air, so that 

 in cooling both sides were positive. 



When the tourmalin was cooled in mercury contain- 

 ed 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 tour- 

 malin to the end of a stick of sealing wax, he allowed 

 it to cool, and then pressed the flat side of it pretty 

 hard against the palm of his hand, and found it to be 

 strongly negative, contrary to what it would have been 

 if exposed to the open air. The stone continued negative 

 till it acquired all the heat that it could receive from his 

 hand, when its power decreased without changing the 

 nature of its electricity. The stone was then allowed 

 to cool in the open air, and it became more strongly 

 negative 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 be- 

 came positive ; allowing it to cool in the open air it 

 became negative ; and by again touching it with hi* 

 hand, it again became positive. In this way he made 

 the same side of the stone alternately positive and ne- 

 gative for a considerable time ; and when the heat was 

 such, that he could keep it in his hand, it acquired a 

 strong positive electricity, which continued till it was 

 brought to the heat of his hand. Analogous results 

 were obtained by fastening the wax to the flat side of 

 the stone, and using the convex side. 



Dr Priestley made a tourmalin very hot, and having 

 covered it all over to the thickness of a crown piece 

 with melted sealing-wax, he found that its electrical 

 virtue was as powerful when enveloped in this coat- 

 ing as when exposed to the open air. Having tied the 

 tourmalin in a silk thread, which only touched the 

 edge of it on both sides, he suspended it before a fire, 

 so that it night be heated equally in every part; when 

 it was so hot that he could hardly bear to handle it, he 

 allowed it to remain in the same situation a quarter of 

 an hour, in order to be certain that the heat was equally 

 diffused. By means of a bundle of fine thread which 

 had for some time been held in the same degree of heat, 

 he took off the electricity which the stone had acquired, 

 and continuing it in the same situation, he found 



