ELECTRICITY. 



457 



Descriptive 



Electricity. 



Effects of 



Wrong heat 

 on the tour 



of attracting and repelling light bodies. The Duke 

 had completely forgotten this circumstance, till the 

 year 1758, when he happened to see some tourmalins 

 in Holland. He immediately purchased these stones, 

 and, in company with Messrs Daubenton and Adanson, 

 he made a number of experiments with them, of which 

 he has published a particular account. 



Before these experiments, however, were made, M. 

 JEpinus had been informed by Lechman of the elec- 

 trical properties of the tourmalin, and had received 

 from him two crystals, on whk-h he made a variety 

 of experiments, which were published in the Memoirs 

 of the Academy of Berlin for 1756, under the title 

 of De quibusdam experiments eleclricis noiabilioribiis. 

 A series of more correct experiments were performed 

 by Mr Benjamin Wilson, with several fine crystals be- 

 longing to Dr Heberden ; and the subject was prose- 

 cuted by Dr Priestley in 1766, with the same tourma- 

 lins which had been used by Mr Wilson. 



The tourmalin crystallizes in prisms usually of nine 

 plane sides terminated by summits, with three, six, 

 nine, or more faces, and are either of a green or blue co. 

 lour. At the ordinary temperature of the atmosphere, 

 the tourmalin may be electrified by friction, and the 

 electricity which it thus acquires is always resinous ; 

 and when two tourmalins are rubbed against each 

 other, the one is electrified positively and the other 

 negatively. 



If we apply different parts of a tourmalin, when ex- 

 cited by a heat between 99^ and 212 of Fahrenheit, 

 (according to -fiipinus), to a delicate electrometer, it will 

 be so excited .is to exhibit two poles coinciding with the 

 summits of the prism, and one of them possessing po- 

 sitive and the other negative electricity. 



If one of the poles of the tourmalin be held near light 

 bodies, such as grains of ashes or saw-dust, these mi- 

 nute bodies will be attracted to the stone, and sometimes 

 repelled as soon as they have touched it. 



When two tourmalins are presented to one another, 

 so that the two positive or the negative pole* are to- 

 wards each other, they will mutually attract one ano- 

 ther ; but if two opposite poles are presented to one 

 another, they will mutually repel each other. In or- 

 der to make this experiment with success, the two crys- 

 tals should be either balanced on a fine pivot, or sus- 

 pended by a delicate fibre, or, what is the simplest me- 

 thod, floated upon two pieces of cork. When the two 

 tourmalins are heated, tie one of them upon a flat piece 

 of cork, and present to one of its poles the two poli-.-: 

 of another tourmalin in succession. When two similar 

 poles are towards each other, the floating tourmalin 

 will turn round and present the opposite pole ; and 

 when two opposite poles are presented to each other, 

 the floating tourmaluVwill follow the other in all its mo- 

 tions, just like a floating needle guided by the action 

 of a magnet. 



Mr Wilson kept a flat tourmalin about half an hour 

 in a strong fire, but could not perceive any diminution 

 of its electrical property, though he repeated the expe- 

 riment with another tourmalin. When he brought the 

 stone, however, to a red heat, and plunged it sudden- 

 ly in cold water, it was not broken to pieces, but had 

 the appearance of being shivered, and lost entirely its 

 electrical jwoperty. M. Hauy, however, has found, that 

 when 115 is more and more heated, there is u 



time when it will cease to yield signs of its electric vir- 

 tue ; and al'trr withdrawing it from the fire, he often 

 found it necessary to leave it to return to a moderate 



VOL. V11I. PAIIT. II. 



temperature, before it exhibited any action upon the 

 little bodies that were presented to it. " It would 

 seem," says that ingenious mineralogist, " that beyond 

 the time where its electricity has become insensible 

 through the action of too strong a heat, there is ano- 

 ther where its effects are reproduced in an inverse 

 sense. We have caused the foci of two buniing glass- 

 es to fall upon the extremities of a tourmalin, and have 

 observed that each pole, after having acquired its or- 

 dinary electricity, would next cease to act, and lastly, 

 would pass to the opposite state, so that the attraction, 

 after having become zero, would give place to repul- 

 sion or reciprocally." 



The experiments made by Canton, respecting the ef- 

 fect of heat upon the tourmalin, differ from those of Mr 

 Wilson, and the Abbe Hauy. Having put a tourma- 

 lin of the common colour into the fire, and burnt it 

 white, he found that its electrical property was com- 

 pletely destroyed. Another tourmalin, heated in a si- 

 milar manner, lost only part of its electricity. Two 

 tourmalins when softened by heat, were joined toge- 

 ther without losing their electrical property. The po- 

 larity of another was increased by having one of its 

 ends melted ; and another tourmalin retained its electri- 

 cal property, after being plunged into cold water when 

 red hot. 



To the Abbe Hauy we are indebted for a very beau- 

 tiful discovery respecting the tourmalin. He found 

 that the electrical density diminishes rapidly from the 

 summits or poles towards the middle of the crystal, and 

 is almost nothing throughout a sensible space towards 

 the middle of the prism. The greatest density which re- 

 sides in the negative and positive poles, is near the sum- 

 mits. This singular distribution of the electric matter 

 is almost exactly the same as in a cylinder : (See Sect, 

 xi. Art. 8.) In order to observe this property, present 

 the tourmalin to the electrified needle of one of Cou- 

 lomb's delicate electrometers, or to an insulated electri- 

 fied needle, finely balanced upon a pivot, and the needle 

 will always be observed to have a marked tendency to 

 one point of the stone, but when the needle points to 

 the middle of the prism, so thut it is equidistant 

 from the two poles, the needle will have no motion ex- 

 cept a mere fluttering. The following curious experi- 

 ment is given by the Abbe Hauy. " Let T," says he, 

 " be a tourmalin, having its centre of resinous action 

 placed at A, and its centre of vitreous action at a. Take 

 a stick of scaling wax, at the end of which there is fix- 

 ed a silk threat! of about a centimetre, or four and a 

 hali'lines in length, by heating the wax at that end, and 

 inserting one extremity of the thread in the part thus 

 melted. If after having rubbed the sealing wax, in 

 which case the free extremity of the thread will acquire 

 resinous electricity, that same extremity be brought in 

 presence of the point H of the tourmalin, and if, at the 

 same time, the latter be made to receive little altcrn.-.tc 

 motions from right to left, and reciprocally, the thread 

 will be seen to bend itself in a contrary direction 

 to avoid the point It ; and if the stick be brought a 

 little nearer the tourmalin, the thread will incline all 

 at once, by a curvilinear motion towards the point .A. 

 If we afterwards present to the thread the points si- 

 tuated a little beyond A, and all the succeeding ones 

 between that and the opposite extremity U, attrac- 

 tions will bo manifested throughout. Hut if a thread, 

 possessing vitreous electricity be employed, such aa 

 that which should be attached to a glass tube, which 

 had been rubbed, on presenting it towards the cxtre- 



3.M 



Electricul 

 density in 

 different 

 parts of the- 

 tourmalin. 



PLATF 



CCXI,\ 

 Fig. i. 



