462 



ELECTRICITY. 



I'rtcriptivc 

 Electricity. 



thikindof 

 r'ectnatjr. 



K.pfri- 

 menu at 

 Van M- 

 rum and 

 VuiTrtXMt. 

 nrk. 



g*re no electrical signs in this way, but when repented - 

 ick upon its end, against * table, it attrni -toil the 

 silk threads. Sulphur produced the same effect. \\lu-n 

 a lump of glass was struck n\ a similar manner against 

 the t..!.i, . it , -\hibited tlu- same electrical effect*. 



No satisfactory result-" have yet been obtained respect- 

 ing the cause of this kind of electricity. It has been ascri- 

 bed, with some reason, to a d. nr ..i friction produ- 

 ced by the contraction which takes place during cooling, 

 or to the circumstance of being touched during the ex- 

 periment. Experiments were made by Messrs Van M.-i- 

 rum and Van Troostwryk, for the purpose of ascertain- 

 ing this point ; and they found that when the bodies 

 were cooled under circumstances which prevented all 

 friction, no electrical phenomena were produced. 



Mer> \ .m Marumand V r an Troostwryk repeated the 

 experiments of Wilcke and .Kpinus, and made use of 

 sulphur, sealing-wax, gum lac mixed with a little rosin 

 to make it melt, rosin, pitch, and wax. They poured 

 nil these substances, when melted, upon the Mirf.ice 

 of' mercury. All of them, except the sulphur, gave 

 strong indications of electricity after they were ta- 

 ken away from the surface of mercury. In order to 

 determine if these bodies became electrical by losing a 

 portion of their natural quantity of fluid, they melted 

 gum lac mixed with rosin and pitch," in vessels of ba- 

 ked clay, and having insulated these vesseb, made a 

 communication 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 be- 

 fore between them and the condenser, but no marks of 

 electricity appeared. A plate of gum lac had one of its 

 surfaces melted over hot coals, and cooled in contact 

 with the condenser, but no electricity was visible. M. 

 Van Marum and his friend now conjectured that the 

 electricity was occasioned by the friction of the fluid bo- 

 dies produced in dispersing themselves over the surface 

 of the body on which they were cooled. For the pur- 

 pose of examining this notion experimentally, they pour- 

 ed 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 that which they 

 would have received if rubbed with the body upon 

 which they were poured. 



If this opinion was well founded, they imagined that 

 in melting plates of any electrical substance, the inte- 

 rior surface of each plate would be more strongly elec- 

 trified than the upper one, as it is the under one alone 

 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, they 

 allowed them to come in contact with the meited gum 

 lac, without any friction being produced. After the 

 from lac was cooled, the plates were raised, but no elec- 

 tricity was exhibited. 



From these experiments their ingenious authors con- 

 elude, that the electricity is produced by the friction Electricity, 

 which the electrical bodies undergo when they are '"^>~ 

 spread upon the surfkees of other bodies upon which 

 they are poured when in a liquid state. 



M. Ch.-iptal discovered, that electricity was developed 

 during the congelation of glacial phosphoric acid. The 

 sa'iie 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 ; 

 Wilcke's Ditpvtatio I'hysii-a Ex^riinenlaitf dc Elcctri* 

 dlali/ms ronlrariis, Rostock 1 757 ; jEpinus Tuntamen 

 T/ieoriie Electricitatis et Magnetismi, IVtrop. 17.58, p. 

 66, 67 ; Henley, Phil. Trans. 1777, p. 85 ; Van Marum 

 et Van Troostwryk Sur la Cause de FElcctricitc det 

 substances fondues et refroidies, in 11" n T\ Observa- 

 tions siir la Physique, 1788, torn. S3, p. 248 ; Liphardt, 

 Sur I' Eleclricite au Chocolat et qiielques objrts relatif, 

 in Hosier's Observations, &c. 1787, vol. xxx. p. 431 

 433 ; Pabst, Annales de Chimie, 1781., p. 119. 



SECT. III. On the Electricity produced by Evaporation. 



We have already seen in our History of the Science, Electricity 

 that Laplace, Lavoisier, and Yolta discovered that elec- P"* 1 "' 

 tricity was generated during the evaporation of various J^^ 

 fluids. 



In their experiments they made use of two different Experi- 

 kinds of apparatus. In both of these, the bodies that menu of 



were evaporated were insulated bv means of stands of }'*P~! 



i r , . . ,. -..-, , ,. Lavomer. 



glass coated with sealing wax. \\ hen the disengage- 

 ment or the absorption of the electric matter was belie\ - 

 ed to be instantaneous, the body was made to communi- 

 cate directly with the electrometer by means of a 

 chain or a thread of archil ; but when the electric mat- 

 ter was supposed to be disengaged successively, and 

 continued for a certain time, they made use of Volta's 

 condenser, which has the property of accumulating 

 small quantities of electric matter. 



Having put iron filings into a vessel with a large 

 aperture, they poured upon them sulphuric acid weaken- 

 ed with three parts of water. After a brisk efferves- 

 cence, and the rapid disengagement of inflammable air, 

 the condenser was so powerfully electrified, that it af- 

 forded a lively spark, and they found from the electro- 

 meter that the electricity was negative. 



When sulphuric acid was, in like, manner, thrown 

 into some vessels that contained powdered chalk, fixed 

 air was rapidly disengaged, and the condenser and elec- 

 trometer indicated that the electricity was negative. 

 It was, however, less than in the preceding experiment, 

 and produced no sensible spark. 



When nitrous gas was produced, they obtained a simi- 

 lar result ; but in order to increase the electricity, they 

 used six vessels at once, each Containing iron filings, and 

 they poured over them nitrous acid diluted with about 

 two parts of water. The effervescence and the produc- 

 tion of air were extremely rapid, and negative elec- 

 tricity was distinctly produced ; but as the circumstances 

 under which the experiment was made were not favour- 

 able, the electricity was very weak. Three small cha- 

 fing dishes filled with burning charcoal were next insu- 

 lated, and made to communicate with Volta's condenser. 

 The negative electricity which was generated was So 

 sensible, that by augmenting the quantity of charcoal 

 they coultl easily have obtained from it a spark. 



The next experiments were made upon the evapora- 

 tion of water, produced by throwing it upon three 

 insulated furnaces of hammered iron, which communi- 

 cated with the electrometer. In three successive ex- 

 periments the electricity was distinctly perceptible. In 



