Saussure found it more difficult to explain why silver sometimes gives a positive electricity. He conjectured, however, that it arose from foreign bodies, such as cop per or other oxidablc metals being mixed with the silver. This conjecture originated in his observing, that though the crucible was extremely clean, and the distilled wa ter very pure, yet when the water was 'educed to a drop, by evaporation, the (Imp always appeared black est, and left a brown or black globule after the evapo ration Was completed.
In order to verify this explanation, he boiled spi rit of salt in his silver crucible, and after having washed it with much care, he found that the electricity which it produced was always negative, even when he projected half an ounce of water into the crucible.
He also observed that when quartz, brought to a white heat, was plunged in water, positive electricity was produced ; a result which he ascribes to a small quantity of iron which may have existed in the quartz.
The production of negative electricity by burning charcoal greatly perplexed our author. In his first ex periments he found the electricity to be positive, but observing that Volta had found it to be negative, he discovered the cause of his mistake, and afterwards obtained the same results as those of Volta. lie sup poses that it may arise from the readiness with which that substance loses its heat in contact with water.
Saussure endeavoured to obtain electricity from com bustion, but his attempts were fruitless, although Volta had obtained distinct indications of it in similar cases. He burned different bodies upon an insulated chafing dish, sometimes with a clear, and sometimes with a smoking flame ; and he endeavoured suddenly to ex plode small heaps of gunpowder, but no electricity whatever appealed.
His attempts to procure electricity without ebulli tion were equally fruitless. He exposed great surfaces, such as six square feet of wet linen, before a large fire, and insulated it by silk cords. \Vhen the linen was strongly heated, a greater quantity of vapour was pro duced than in a coffee-pot boiling on a chafing-dish, and though he used the most delicate electrometer, yet no electricity could be seen. He likewise spread out on thr-moist ground a large plate of white iron, and heated it strongly on a chafing-dish, but though the earth exhaled a great quantity of vapour, no elec tricity was produced. See La Place and Lavoisicr, Mem. ?cad. Par. 1781, p. 292 ; Volta, Journal (le Phy sique, 1783; and Saussure, Voyages dans les .411tes, 8vo, edit. 1786, tom. iii. p. 316-51.7.
One of the most singular electrical phenomena which belongs to the present Section, is the evolution of dce tric light by the bursting of the unannealed glass tears, called Prince Rupert's drops; a fact which was disco vered by Dr Brewster. These drops, which he found to possess the property of transpolarising light, were regularly crystallized, and had three different cleavages : one like that of a melon, diverging from the apex of the drop; another concentric with the surface of the drop; and a third oblique to the axis. having laid one of
these drops upon a table in a dark room, and covered it with a plate of thick glass, to prevent any of the fragments from reaching the eye, the drop was burst by breaking of a part of its tail, and the whole of it appeared luminous; so that at the instant of the fracture, a quantity of faint light, of the same shape and size as the drop itself, was distinctly visible. The drop which gave this singular result was made of flint glass, and was the largest that he had ever seen. Eve ry other flint glass drop produced a distinct electrical light ; but in none of them, except the large one, could he sec the luminous shape of the drop. The same light appeared v lien they were burst under water. The small glass drops made of bottle glass never exhi bited any light at the moment of bursting; but it was almost always visible, in small sparks, in bottle glass drops of a larger size.
A similar phenomena is exhibited in Muscovv talc, when its laminze are suddenly torn asunder. Electrici ty is immediately developed, and a bright Hash of light is exhibited.
Mr William Wilson found, that if a piece of dry and warm wood is suddenly split asunder, the two conti guous surfaces are electrified, the one positively and the other negatively.
\Vhen a stick of sealing wax is broken in two, the two fractured extremities arc distinctly electrified, the one end positively and the other negatively.
Connected in soine measure' with the preceding- re sults, are those which were obtained by Mr Wilsou respecting the electricity of wood shavings. Having had frequent occasion to work very dry wood that had lain for several hours over a Liege lire, Mr Wilson often observed the shavings adhering to the tools. and to every thing that they touched. Ile therefore instituted a set of experiments, for the purpose of as certaining the origin of the electricity, and obtained the following general results. When dry wood was scra ped with a piece of window glass, the shavings always exhibited positive electricity. When the wood was chipped with a knife, the electricity of the chips was positive when the wood was hot, and the edge of the knife not very slim p, but negative when the w ood was perfectly cold. \Vhen the edge of the knife was very sharp, the chips were negatively eleetrifiCd whatever was the temperature of the wood. Having insulated a penknife, by fixing it into a glass tube covered with sealing wax, he found that it always possessed an electri city contrary to that of the chips, which were most fre qucntly positive. The surface of the wood from which