Exp. 11. If a plane. not insulated, is placed at any distance from an electrified globe, so that the electricity cannot be communicated from the one to the other but across the stratum of air which separates them, then Coulomb found that the electrical densities of a point in the centre of the plane, is of a nature contrary to that of the globe, and is inversely as the square of its distance from the centre of the globe.
On the Phenomena of Electricity produced TVith out citation.
Ix the preceding Chapter we have endeavoured to give a succinct view of the leading phenomena of elec tricity as produced by friction : We shall now proceed to direct the attention of the reader to a series of inter esting electrical phenomena as produced without fric tion, either by a change of temperature, by a change of form, or by the contact of dissimilar bodies; or as exhi bited in the phenomena of the atmosphere, or in the functions of living animals.
SEcer. I. On Electrical Phenomena produced by a change Temperature.
THE property of exhibiting eh:oilers) phenomena merely by an increase of temperature, without the aid of friction, is possessed only by regularly crystallized minerals. The tourmalin was, for a long time, the only substance which was known to he capable of this kind of excitation ; but the seine property has since been re cognized in the topaz, in calamine or the oxide of zinc, in the borate of magnesia, and in mesotype.
1. On the Electrical Properties of the Tourmalin.
The electrical properties of the tourmalin seem to have been known to the ancients. The Ly11C117111111 is mentioned by Thcophrastus, as being a very hard body, as being used for making seals, as requii inn; a great la bour to polish it, and as possessing On: same property as amber of attracting light bodies: It is ther•fk.re highly probable that this substance was the tonrinalin of ino Bern mineralogists.' In the island of Ceylon, where it is very common, it is known by the name of tournamal ; and the Dutch, who first became acquainted with it in this island, gave it the appellation of Aschentrikker, from its property of attracting ashes when it is thrown into the fire.
In the year 1717, Alons. Lemery of the Academy of Sciences, exhibited a stone from Ceylon, which he said attracted and repelled different light bodies, such as ashes, filings of iron, bits of paper, &-c. in a manlier dif
ferent ft oin a loadstone.f The experiments of Lemery were merely noticed by Linnaeus in his Flora Zeylonica, who mentions this stone by the name of la/us electricus.
When the Duke de Nova was at Naples in the year 1743, he was informed by the king's secretary, Count Pichetti, that he had seen at Constantinople, a small stone, called tourmalin, which had the singular faculty .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 com pany with Alessrs Daubenton and Arlanson, he made a number of experiments with them, of which he has published a particular account.
Before these experiments, however, were made, M. Epinus had been informed by Lechman of the electrical properties of the tourmalin, and had received from him two crystals, on which he made a variety of experi ments, which were published in the Memoirs of the Academy of Berlin for 1756, under the title of De quibusdam exfierimentis electricis notabilioribus. A se ries of more correct experiments were performed by Air Benjamin Wilson, with several fine crystals belong ing to Dr Heberden; and the subject was prosecuted by Dr Priestley in 1766, with the same tourmalins 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 colour. At the ordinary temperature of the atmosphere, the tourmalin may be elecuified by friction, and the elec tricity 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 tourinalin, when ex cited by a heat between 991° and 212° of Fahrenheit, (according to /Epinus), to a delicate electrometer, it will be so excited as to exhibit two poles coinciding with the summits of the prism, and one of them possessing positive and the other negative electricity.