GALVANISM. 



3 



acid from the acid to the copper 

 from the copper back again to the zinc 

 and so on, in a perpetual circuit. 

 Such at least must be the explanation 

 of the phenomena on the hypothesis of 

 Franklin, implying the singleness of the 

 electric fluid. But if the theory of Du 

 Fay, which recognises two different 

 fluids, be adopted, what has just been 

 stated must be understood to refer ex- 

 clusively to the current of vitreous elec- 

 tricity. Now, according to that theory, 

 every such transfer of electricity con- 

 sists of an interchange of the two fluids : 

 the current of vitreous electricity just 

 mentioned, must, therefore, necessarily 

 be accompanied by an opposite current 

 of resinous electricity ; that is, of one 

 flowing from the zinc to the copper; 

 from the copper to the acid ; and from 

 the acid to the zinc. Hence in our fu- 

 ture explanations of the phenomena of 

 galvanism, it will be sufficient to ex- 

 press the former of these currents only ; 

 provided we bear in mind that the trans- 

 fer of any quantity of vitreous electricity 

 in a given direction, implies the transfer 

 of an equal quantity of resinous electri- 

 city in the opposite direction. 



(5.) The same effects will take place, 

 if, instead of allowing the metallic plates 

 to come in direct contact, the communi- 

 cation betwen them be effected by wires, 

 (as shewn in fig. 2.) extending from the 



Fig 2. 



one to the other. The circuit of elec- 

 tricity will thus be lengthened, but the 

 currents will move in the same direction 

 as before ; that of the positive electri- 

 city being denoted in the figure by the 

 position of the arrows ; namely, in the 

 fluid, from the zinc towards the copper ; 

 and along the wires, from the copper to 

 the zinc. The completion of the circuit 



by means of wires, enables us to 'direct 

 the electric current through such bodies 

 as we may wish to subject to its opera- 

 tion, and at the same time gives us the 

 power of interrupting or renewing at 

 pleasure the communication between the 

 two metallic plates, by merely separat- 

 ing or joining together their remote ex- 

 tremities at Y. When united, the wire 

 W, which proceeds from the copper- 

 plate C, is imparting electricity to the 

 wire X, which touches the zinc plate Z ; 

 hence, the former is considered as being 

 in a positive, and the latter in a nega- 

 tive state. 



(6.) The electrical effects of the sim- 

 ple apparatus just described are, in 

 general, too feeble to be perceived, un- 

 less by very delicate tests. The fact 

 mentioned by Sulzer, and the experi- 

 ments of Galvani on the muscles of 

 frogs, in their original form, afford, 

 however, examples of the operation of 

 simple galvanic circles. When the 

 tongue is interposed between zinc and 

 copper, the saliva in contact with the 

 metals performs the part of the acid in 

 the experiment above mentioned, and 

 the stream of electricity in its passage 

 from the zinc to the copper, through 

 the substance of the tongue, affects the 

 nerves of that organ, so as to give rise 

 to sensations of taste. In Galvani's 

 experiment, muscular contractions were 

 produced by forming a connection be- 

 tween two different metals, one of which 

 was applied to the nerve, and the other 

 to the muscles of a frog's leg. It is 

 evident that such an arrangement com- 

 poses a galvanic circle, deriving its 

 activity from the chemical properties of 

 the fluids in those parts of the frog that 

 are in contact with the metals. Al- 

 though the quantity of electricity set in 

 motion by this slight action, must be 

 supposed to be exceedingly minute, it is 

 yet sufficient, when passing over the 

 exquisitely sensible nerves of the tongue, 

 or through the highly irritable fibres of 

 a frog, to produce a very considerable 

 impression. 



(7.) It has even been found possible, 

 by means of a very small galvanic circle 

 of the same simple kind as that which, 

 we have described, to produce some of 

 the more energetic effects of galvanism, 

 such as raising the temperature of the 

 wire which conducts it to a red heat. 

 We are indebted to the ingenuity of Dr. 

 Wollaston for the contrivance of an ap- 

 paratus, which he calls an elementary 

 galvanic battery, capable of exhibiting 

 u 2 



