M. Liphardt made also some experiments on the elec trical effects produced by a sudden blow. He let fall a piece of sealing-wax, from the height of 8 inches, upon a table, and repeated this from 10 to 20 times, when it exhibited marks of electricity. Gum copal gave no electrical signs in this way, but when repeatedly struck upon its end, against a table, it attracted the silk threads. Sulphur produced the same effect. When a lump of glass was struck in a similar manner against the table, it exhibited the same electrical effects.
No satisfactory results have yet been obtained re specting the cause of this kind of electricity. It has been ascribed, with some reason, to a degree of friction produced by the contraction which takes place during cooling, or to the circumstance of being touched during the experiment. Experiments were made by Messrs Van Marum and Van Troostwryk, fur the purpose of ascertaining this point; and they found that when the bodies were cooled under circumstances which prevent ed all friction, no electrical phenomena were produced.
Messrs Van Martial and Van Troostwryk repeated the experiments of Wilke and /Epinus, and made use of sulphur, sealing-wax, gum lac mixed with a little ro sin to make it melt, rosin, pitch, and wax. They pour ed all these substances, when melted, upon the surface of mercury. All of them, except the sulphur, gave strong indications of electricity after they were taken away from the surface of mercury. In order to deter mine if these bodies became electrical by losing a por tion of their natural quantity of fluid, they melted gum lac mixed with rosin and pitch, in vessels of baked clay, and having insulated these vessels, made a communica tion between the melted bodies and Volta's condenser, which was very sensible. No electricity, however, was produced, although the experiment was thrice repeated.
In order to shew that the vessel of baked clay had not acquired the contrary electricity, they poured the elec trical substances upon linen and upon gauze suspended by silk cords, and as soon as the bodies had lost their fluidity by cooling, a communication was made as before between them and the condenser, but no marks of eke trinity appeared. A plate 01 won i.e had one of it' surfaces melted over hot coals, and c, in °nun with the condenser, hut no electricity a as risible.
Van Martun and his friend now conjectured, that ti.e electricity was occasioned by the f•iLtiou of the Iluid L. dies produced in dispersing themselves o% er the surt.,, .
of the body on which they were cooled. Fur the put pose of examining. this notion experimentally, tiny poured the melted bodies upon copper, tin, lead, glass, and English earthen-ware, and they always found that they acquired the same kind of electricity as thz.t ulacit they would have received if rubbed with the body upun which they were poured.
If this opinion was well founded, they imagine d that in melting plates of any electrical substance, the rior surface of each plate would be more strongly ch. e trifled than the upper one, as it is the under one alma: that undergoes the friction. This trial was made with plates of gum lac and rosin, an inch thick. and they al ways found that the separation of the balls of Cavallo's electrometer was twice as great with the electricity of the lower surface, as with that of the upper one. They also tried plates of various thicknesses, some of which were an inch and a quarter thick, and in no case did they perceive any difference arising from a difference of thickness.
In order to ascertain this point with still more cor rectness, they melted gum lac and rosin, and having suspended plates of copper by means of silk cords, flit y allowed them to conic in contact with the melted gum lac, without any friction being produced. After the gum lac was cooled, the plates were raised, but no electricity was exhibited.
From these experiments their ingenious authors con clude, that the electricity is produced by the bit tion which the electrical bodies undergo when they are spread upon the surfaces of other bodies upon which they are poured when in a liquid state.
M. Cliaptal discovered, that electricity was developed during the congelation of glacial phosphoric acid. The same property has been found in calomel, when it fixes by sublimation to the upper part of a glass vessel.
See Gray, Phil. Trans. 1732. vol. xxxvii. p 285 ; \Vilke's Dispzaatio Physica Experimentalis de Electri eitatibus contrariis, Rostock 1757 ; ,Epirus Ten'anzen Theorix Eleetricitatis et Illagnetiszni, Petrol). 1758, p. 66. 67 ; Henley, Phil. Trans. 1777, p. 85 ; Van :\ larton ct Van Troostwryk Sur la Cause de l'Electricitc des substances fondues et rzfroidies, in lluzitr's Observa tions sur la Physique, 1788, torn. 33, p. 248 ; 1,;pnardi, Sur l'Electricite du Chocolat Cr quelques objets relatil in Rozier's Observations, Sec. 1787, vol. xxx. p. 431-433; Pabst, .4nnales Chimie, 1784, p. 119.