VOLTAISM. 



tranfmilted from the zinc to platina, during the chemical 

 aftion, many experiments feem to prove ; and that the hy- 

 drogen fo tranlmitted, by its chemical agency, and in its 

 nafcent ftate, is capable of effeding many decompofitions, 

 which, under other circumitances, would be impoffible. 

 In the fingle combination above alluded to, if the dilute acid 

 be feparatcd from a folution of acetate of lead, or fulphate 

 of copper, by a piece of bladder, the zinc being immerfed 

 into the acid part, and the platina into the metallic folution, 

 no hydrogen will be afforded by the platina, but the metal 

 becomes reduced in proportion to the quantity of hydrogen 

 which has difappeared : yet no perceptible quantity of this 

 cffeft can be attributed to the eleftricity of contaft, but to 

 the mere chemical agency of hydrogen in its nafcent ilate. 

 Hence we are inclined to think, that the decompofitions by 

 the galvanic battery arife from two caufes. Water princi- 

 pally owes its decompofition to the chemical aftion, and the 

 agency of the eleftricity upon the hydrogen. Metallic 

 oxyds are principally decompofed by the prefence of the 

 nafcent hydrogen, fo coUefted and tranfmitted by the elec- 

 tricity. The decompofition of faline bodies, however, is 

 to be attributed alone to the eleftrical attraftion produced 

 by the contaft of the bodies employed, which can be made 

 fo great as to overcome the chemical attraction of the bodies 

 decompofed. Of the latter of thcfe powers of decompofi- 

 tion we have given fome account, in delaiUng the ingenious 

 experiments of fir Humphrey Davy ; of the two former 

 means of decompofition we fliall fay fomething in a pradical 

 point of view. 



Many very anomalous fafts were known in chemiflry 

 long previous to the difcovery of Galvanifm. All thofe 

 chemical phenomena, under which the appearance called ar- 

 borefcence was obferved, were inexplicable, till it was ihewn 

 from fome experiments, publilhed in Nicholfon's Journal, 

 vol. XV. p. 94, that Galvanifm is the caufe of tliefe Angular 

 phenomena. In the experiment where lead is fo beautifully 

 precipitated, by fufpending a piece of zinc in a folution of 

 acetate of lead, the zinc firft reduces a fmall portion of 

 lead, which, with the zinc, forms a galvanic combination. 

 The lead, if no folution of lead were prefent, would now 

 give out hydrogen gas; but the hydrogen, in (lead of ap- 

 pearing in that form, combines with the oxygen of the 

 oxyd, and the metallic lead is formed at the fame point. 

 Hence the lead appears to grow from the laft. point formed, 

 which gives the appearance of vegetation. That this effect 

 does not depend upon the prefence of zinc, may be proved 

 by the following experiment. Tie on one end of a glafs 

 tube, about half an inch wide, a piece of bladder, fo that 

 it may hold water, and fill it with a folution of acetate of 

 lead. Into-the other end infert a cork loofely, and through 

 the -cork let a platina wire pafs within about half an inch of 

 the bladder. Into a wine-glafs put fome dilute muriatic 

 acid, in which place a zinc wire. When the tube with the 

 bladder is immerfed in the wine-glafs, if that part of the 

 zinc wire without the glafs be brought into contatt with that 

 part of the platina wire without the tube, beautiful cryftals 

 of metallic lead will foon appear upon the platina wire. If 

 the acetate of lead be removed, and a dilute acid be put in 

 its place, bubbles of hydrogen will appear upon the platina 

 wire. 



Another experiment, fimilar to that of the lead-tree, and 

 equally anomalous, has been long known in chemiftry. If 

 a plate of glafs be fmeared over with a folution of nitrate of 

 filver, and a brafs pin or a piece of zinc wire be laid in the 

 middle of the plate, beautiful ramifications of filver will foon 

 appear as if growing out of the pin, very much refembling 



vegetation. By obferviiig the procefs with a magnifying 

 glafs, each branch of this arborefcence may be feen to grow 

 from tlie end or fide of another ; which proves that the filver 

 forming the vegetative appearance is not reduced by the 

 oxydable metal laid on the plate, but by fomething at the 

 fucceffive points of the filver branches. With a view to 

 afcertain this faft, one half of the plate fhuuld be fmeared 

 with nitrate of filver, and the other half with dilute muriatic 

 acid. If a piece of zinc wire be tied to a piece of platina 

 wire, and the compound wire fo bent that the zinc may 

 touch the dilute acid, and the platina the nitrate of filver, 

 the ramifications of filver will foon appear upon the platina 

 wire. That the filver is reduced by the hydrogen carried 

 in the galvanic current, is probable from varying the ex- 

 periment as follows : If, inilead of fmearing the plate 

 with nitrate of filver, the whole be covered with dilute acid, 

 and the fame compound arc be laid upon it, the platina will 

 give out bubbles of hydrogen. In the common way of 

 making this experiment with the pin, as well as the variation 

 above ftated, it appears that the procels is kept up by the 

 galvanic current, which furnilhes the hydrogen. The pin 

 firll reduces a fmall portion of filver, which forms a gal- 

 vanic combination with the pin. The hydrogen which, but 

 for the prefence of the remaining nitrate of filver, would 

 appear in the gafeous form, is employed in depriving the 

 filver of its oxygen. With the compound arc, the zinc 

 does not require to touch the nitrate of filver, becaufe the 

 platina with zinc is already a galvanic combination. The 

 theory of whitening common pins can be explained only on 

 this principle. Tlie tin, in a fmall proportion, is diffolved 

 in the tartrate of potafli ; pieces of metallic tin, with the 

 pins, are alfo prefent. The two latter form the galvanic 

 combination, and a portion of tin is reduced from the folu- 

 tion upon tlic pins, to which they owe their whitenefs. We 

 may generally conclude, that in all inflances where one 

 metal becomes the precipitant of another, the precipitation 

 is much facihtated by the agency of the galvanic com.bina- 

 tion, formed between the precipitating and the precipitated 

 metals, and the confequent prefence of hydrogen. If a 

 piece of zinc be introduced into a folution of fulphate of 

 copper, the zinc in the firfl inilance becomes covered with 

 copper, and the effeft appears to flop. If, however, a 

 very fmall excefs of fulphuric acid be added, the procefs 

 will go on with fuch rapidity, that the copper becomes 

 precipitated in a very Uttle time. By minutely obferving 

 the procefs, the copper will be feen to be reduced upon 

 that already produced, which is a proof that it is not done 

 by the mere agency of the zinc. 



It appears very evident, that when a galvanic combina- 

 tion of zinc with any leffer oxydable metal is placed in a 

 dilute acid, that a much larger quantity of hydrogen will be 

 evolved from the leffer oxydable wire, than could pofTibly 

 be produced by any eleftrical intenfity generated by the 

 contaft of the bodies employed ; but that independent of 

 this, there is an immenfe quantity of eleftricity generated 

 during the chemical aftion, by which the hydrogen is tranf- 

 ported from the greater oxydable furface to the leffer one. 

 If the quantity of hydrogen produced depended upon the 

 attradlioil of the wires for the elements of the water, this 

 power would depend upon the eleftrical intenfity alone, and 

 of courfe upon the feries in the galvanic battery, whatever 

 might be its furface ; but it is found that the po—--r -*' Gal- 

 vanifm to decompofe water is much increafed by an increafe 

 of furface only. 



Galvanifm as a Source of Heat. — When the wires coming 

 from the ends of a galvanic battery of coafiderable furface 



are 



