56 



ELECTRICITY. 



tricity. Thus it was established as a 

 general fact, that these two metals, in- 

 sulated, and in their natural state, are 

 brought, by mutual contact, into oppo- 

 site electrical states ; the zinc acquiring 

 positive electricity, and the copper be- 

 coming, in an equal degree, negative. 



(204.) No explanation has yet been 

 given of this curious fact, which seems 

 to be at variance with all the previously 

 ascertained laws of electric equilibrium. 

 The transfer of electricity from one me- 

 tal to the other during their contact, im- 

 plies the operation of some new force 

 which no theory has yet embraced. 

 While the contact is preserved, neither 

 of the metals gives any indication of its 

 electrical state, the electricity being dis- 

 guised ; as would be the case of that of 

 the coatings of a Leyden jar, if we could 

 suppose them both in actual contact, 

 but yet incapable of allowing any trans- 

 fer of the electricity from the one to the 

 other, so as to restore both to the state 

 of neutrality. 



We shall have occasion to resume the 

 consideration of this curious subject in 

 the treatise on Galvanism, with the 

 theory of which it appears to have an 

 intimate relation. 



(205.) There are some bodies which 

 are rendered electrical by pressure. This 

 property is possessed in the most re- 

 markable degree by that transparent 

 variety of carbonate of lime which is 

 known by the name of Iceland spar. 

 According to Hauy, if a crystal of this 

 spar, which has the form of a rhomboid, 

 be held in one hand by two of its oppo- 

 site edges, and if at the same time two 

 of its parallel planes be lightly touched 

 by two fingers of the other hand, and 

 then brought near to the small needle of 

 the electroscope, ( 12) a decided attrac- 

 tion will be perceptible. By applying a 

 more powerful pressure, the electrical 

 effects will be still more considerable ; 

 the electricity being in all cases positive. 

 Hauy observes that this property resides 

 principally in those crystalline minerals 

 that are capable of being reduced by 

 mechanical division to plane and smooth 

 laminae : such as the Topaz, especially 

 the colourless variety ; Euclase, Arra- 

 gonite, Fluate of lime, and Carbonate of 

 lead. Among those substances in which 

 friction excites negative electricity, there 

 are some which require only to be 

 pressed, for the production of the same 

 effect. An instance occurs in elastic 

 bitumen, when it has been cut into a 

 proper shape for the experiment. Mr. 

 Becquerel has lately discovered that 



many other substances, such as cork, 

 bark, hairs, paper, and wood, possess 

 the property of producing electricity by 

 compression. 



(206.) Many substances, when re- 

 duced to powder, exhibit electricity, if 

 they are made to fall upon an insulated 

 metallic plate. This fact was first no- 

 ticed by Mr. Bennet, after he had in- 

 vented his gold-leaf electroscope. He 

 found that powdered chalk, put into a 

 pair of bellows, and blo\vn upon the cap 

 of the electroscope, communicates to the 

 instrument positive electricity, when the 

 pipe of the bellows is about six inches 

 from the cap ; but the same stream of 

 powdered chalk electrifies it negatively 

 at the distance of three feet. On being 

 blown in a more copious stream from a 

 pair of bellows without the pipe, the 

 electricity is always negative; and the 

 same effect takes place when the powder 

 is let fall from another plate upon the 

 cap of the instrument. This subject was 

 pursued by Cavallo ; but the most com- 

 plete set of experiments relating to it is 

 that of Singer, who employed in his re- 

 searches the two following methods : 

 first, that of sifting the powders on the 

 cap of a delicate electrometer through a 

 fine sieve, which was thoroughly cleaned 

 after each operation ; and secondly, that 

 of bringing an insulated copper plate 

 repeatedly in contact with extensive sur- 

 faces of the powders spread on a dry 

 sheet of paper ; the copper plate being 

 brought in contact with the condenser 

 after every repetition of the contact, 

 until a sufficient charge was communi- 

 cated. 



(207.) The following substances, ac- 

 cording to Singer, produce negative 

 electricity when sifted on the cap of the 

 electrometer: viz. copper, iron, zinc, 

 tin, bismuth, antimony, nickel, black 

 lead, lime, magnesia, barytes, strontites, 

 alumine, silex, brown oxide of copper, 

 white oxide of arsenic, red oxide of lead, 

 litharge, white lead, red oxide of iron, 

 Acetate of copper, sulphate of copper, 

 sulphate of soda, phosphate of soda, 

 carbonate of soda, carbonate of am- 

 monia, carbonate of potash, carbonate 

 of lime, muriate of ammonia, common 

 pearl-ashes, boracic acid, tartaric acid, 

 cream of tartar, oxy muriate of potash, 

 pure potash, pure soda, resin sulphur, 

 sulphuret of lime, starch, orpiment. 



(208.) The following substances pro- 

 duce positive electricity under the same 

 circumstance : viz. wheat flour, oat- 

 meal, lycopodium, quassia, powdered 

 cardamom, charcoal, sulphate of potash, 



