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.
\Vhen two tourmalins are presented to one another, so that the two positive or the negative poles are towards each other, they will mutually attract one another ; but if two opposite poles are presented to one another, they will mutually repel each other. In order to make this experiment with success, the two crystals should be either balanced on a fine pivot, or suspended by a deli cate fibre, or, what is the simplest method, floated upon two pieces of cork. When the two tourmalins are heat ed, tie one of them upon a flat piece of cork, and pre sent to one of its poles the two poles of another tourma lin in succession. \Vhen 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 tournialin will follow the other in all its motions, just like a floating needle guided by the action of a magnet.
Alr 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 experi ment with another tourmalin. When he brought the stone, however, to a red heat, and plunged it suddenly in cold water, it was not broken to pieces, but had the appearance of being shivered, and lost entirely its elec trical property. Al. llauy, however, has found, that when the stone is more and more heated, there is a time When it will cease to yield signs of its electric virtue ; and after withdrawing it from the fire, he often found it necessary to leave it to return to a moderate tempera ture, 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 another where its effects are reproduced in an inverse sense. We have caused the foci of two burning glasses to fall upon the extremities of a tourmalin, and have observed that each pole, after having acquired its ordinary electricity, would next cease to act, and lastly, would pass to the opposite state, so that the attraction, after hating become zero, would give place to repulsion, or reciprocally." The experiments made by Canton respecting the ef fect of heat upon the tourmalin, differ from those of Alr 1Vilson and the Abbe Hauy. Having put a tourmalin of the common colour into the fire, and burnt it white, he found that its electrical property was completely de stroyed. Another tourpialin, heated in a similar manner, lost only part of its electricity. Two tourmalins when softened by heat, were joined together without losing their •electrical property. The polarity of another was increased by having one of its ends melted ; and another tourmalin retained its electrical property, after being plunged into cold water when red hot.
To the Abbe Hauy we are indebted for a very beauti ful discovery respecting the tourmalin. He found that the electrical density diminishes rapidly from the sum mits 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 M the negative and positive poles, is near the sum mits. This singular distribution of tue 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 that it is equidistant from the two poles, the needle Ni ill have no motion except a mere fluttering. The following euritus experiment is given by the Abbe Hauy. " Let T," (Plate CCXLV. Fig. 4.) says he, (c be a tourtznalin, having its centre of resinous action placed at A, and its centre of vitreous action at a. Take a stick of sealing ‘vax, at the cud of w fah there is fixed a silk thread of about a centimetre, or lour and a half 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 scaling wax, in which case the free extremity of the thread ill acquire resinous electricity, that same extremity be brought in presence of the point R of the tourmalin, and if, at the same time, the latter be made to receive little alternate motions from right to left, and reciprocally, the thread will be seen to bend itself In a contrary direction to avoid the point R; 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 after wards present to the thread the points situated a little beyond A, and all the succeeding ones between that and the opposite extremity U, attractions will be manifested throughout. But if a thread, possessing vitreous elec tricity be employed, such as that which should be attach ed to a glass tube, which had been rubbed, on present ing it towards the extremity U, it will avoid going on to touch that extremity by inclining towards the point a ; and all the points situated between a and the extremity R, will act upon it by attraction ; so that we shall not have precisely the inverse of the preceding effects, be cause, in both cases, the thread is attracted by the mid dle part of the tourmalin." If a tourmalin, when in a state of excitation by heat, is broken into two parts, however small, each fragment has two opposite poles, a phenomenon analogous to what takes place in a broken magnet. Mr Canton cut a large irregular tourmalin into three pieces, and a piece being cut from the positive, and another from the negative pole, the outer side of the piece which he cut from the negative pole was negative when cooling, and the outer side of the piece which he cut from the positive pole was positive while cooling, the opposite ends of all the pieces possessing a contrary electricity. The middle piece had the same properties as in the outer tourmalins, the positive end remaining positive, and the negative 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 accident 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.