ELECTRICITY. 



57 



nitrate of potash, acetate of lead, oxide 

 of tin. 



(209.) The following catalogue ex- 

 hibits the results of the experiments of 

 contact with a copper plate ; the dif- 

 ferent substances being arranged under 

 the head of the electricity they really 

 acquire, which is contrary to that of the 

 copper plate. Positive : lime, barytes, 

 strontites, magnesia, pure soda, pure 

 potash, common pearl-ashes, carbonate 

 of potash, carbonate of soda, tartaric 

 acid. Negative : benzoic acid, boracic 

 acid, oxalic acid, citric acid, silex, 

 alumina, carbonate of ammonia, sul- 

 phur, resin. These experiments were 

 several times repeated with uniform 

 results. 



(210.) The above mode of electrical 

 excitation is probably merely a species 

 of friction, differing only from the more 

 ordinary instances by the mode of its 

 application. But in other cases the 

 electrical effects of contact are more 

 distinctly exhibited, as when zinc filings 

 are poured through holes in a plate of 

 copper, upon the cap of an electro- 

 meter. 



(211.) The following experiment, 

 founded on one devised by Professor 

 Lichtenberg of Gottingen, is an elegant 

 illustration of the opposite electrical 

 states of different powders. With the 

 knob of a charged jar, trace on the 

 surface of a smooth plate of glass, or of 

 any resinous substance, various lines at 

 pleasure ; and then repeat the same 

 operation in other parts with the knob 

 of a jar charged with the opposite 

 electricity. Let the surface thus pre- 

 pared be gently dusted, by means of a 

 powder-puff, with a mixture of pow- 

 dered sulphur and red lead, previously 

 triturated together in a mortar. By the 

 contact and friction thus produced, the 

 sulphur has been rendered negative, 

 and the red lead positive ; and each of 

 the powders, when projected on the 

 plate, will attach itself to the oppositely 

 electrified lines, forming a series of red 

 and yellow outlines. It is also observ- 

 able, that the configurations assumed 

 by these and other powders differ ac- 

 cording to the species of electricity im- 

 pressed upon the plate ; positive elec- 

 tricity producing an appearance resem- 

 bling feathers, and negative electricity 

 an arrangement more like stars. 



(212.) The most important circum- 

 stance in this inquiry, is the connection 

 between electricity and the chemical 

 properties of matter. It is observed by 

 Sir H. Davy, that most of the sub- 



stances that act distinctly upon each 

 other electrically, are likewise such as 

 act chemically, when their particles 

 have freedom of motion : this is the 

 case with the different metals, with sul- 

 phur and the metals, with acid and 

 alkaline substances. Of two metals in 

 contact, the one which has the greatest 

 chemical attraction for oxygen acquires 

 positive electricity, and the other the 

 negative : so that if arranged in the 

 order of their oxidability, as follows, 

 zinc, iron, tin, lead, copper, silver, gold, 

 platina, each will become positive when 

 brought into contact with any that fol- 

 low it in the series, and negative with 

 any of those which precede it. In con- 

 tacts of acids with bases, as of crystals 

 of oxalic acid with dry quicklime, the 

 former is negative, the latter positive. 

 All acid crystals when touched by a 

 plate of metal render it positive, the 

 crystals themselves becoming negative. 



(213.) Bodies that exhibit electrical 

 effects by mutual contact, previous to 

 their chemical action on each other, 

 lose this power during combination. 

 Thus if a polished plate of zinc be made 

 to touch a surface of dry mercury, and 

 quickly separated, it is found positively 

 electrical, and the effect is increased by 

 heat ; but if it be so heated as to amal- 

 gamate, that is, unite chemically with 

 the mercury, it no longer exhibits any 

 signs of electricity. The case is analo- 

 gous with copper and sulphur ; and 

 iron, when applied to mercury, produces 

 more electricity than zinc, apparently 

 from its being incapable, under ordinary 

 circumstances, of forming a chemical 

 combination with mercury. 



(214.) On the other hand, there can 

 be no question that electricity is oc- 

 casionally, if not universally, elicited 

 during chemical action. We have just 

 seen that a dry acid becomes negative 

 by contact with a metal, which is con- 

 sequently thereby rendered positive. In 

 this case no chemical combination had 

 taken place. But Becquerel has shown 

 that if the acid, instead of being in a 

 dry crystalline form, be in a liquid state, 

 and capable of acting chemically on the 

 metal, the acid will become positive and 

 the metal negative. The same conclu- 

 sion may also be deduced from the 

 experiments of Lavoisier and Laplace, 

 on the action of dilute sulphuric acid 

 on iron filings. That the oxidation of 

 metals gives rise to electricity has been 

 also shown by the experiments of Dr. 

 Wollaston, from which it would appear 

 that the electricity obtained in the .com- 



