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GALVANISM. 



two elements. As the conducting power 

 of fluids, however, is much inferior to 

 that of metals, the electrical indications 

 will be more feeble than in circles of 

 the first class ; and, indeed, will scarcely 

 be sensible unless we employ the more 

 easily oxidable metals, such as tin and 

 zinc. But powerful effects may be ob- 

 tained by combining a number of such 

 circles in a pile or battery. For con- 

 structing an instrument of the for- 

 mer kind, Sir H. Davy directs pieces of 

 polished tin, about an inch square and 

 one-twentieth of an inch thick, to be 

 piled up with woollen cloths of the 

 same size, moistened some in water, 

 and some in dilute nitric acid, in the 

 following order, tin, acid, water, and so 

 on. It is proper to observe the pre- 

 caution of placing the cloth moistened 

 with acid underneath the one which is 

 moistened with water ; for, as the acid 

 is specifically heavier than the water, 

 little or no mixture of fluid will then 

 take place. Twenty such alternations 

 will produce a battery capable of acting 

 weakly on the organs of sense, and of 

 slowly decomposing water. When zinc 

 is the metal used, it is necessary, on 

 account of its rapid oxidation in water 

 containing atmospheric air, to use three 

 cloths ; the first moistened with a weak 

 solution of hydro-sulphuret of potash, 

 which has no power of acting upon 

 zinc, and which prevents it from being 

 acted upon by the water; the second 

 moistened with a solution of sulphate 

 of potash, of greater specific gravity 

 than the solution of hydro-sulphuret ; 

 and the third wetted with an oxidating 

 fluid, such as an acid, specifically hea- 

 vier than either of the solutions. In 

 this case, if, proceeding upwards, the 

 order be as follows zinc, oxidat- 

 ing solution, solution of sulphate of 

 potash, solution of hydro-sulphuret of 

 potash, very little mixture of the fluids, 

 or chemical action between them will 

 take place ; and an alternation of twelve 

 series of this kind, forms a battery ca- 

 pable of producing sensible galvanic 

 effects. The direction of the electrical 

 current is, as usual, from the zinc to 

 the oxidating fluid. 



(80.) It has often been remarked that 

 porter drank out of a pewter pot has a 

 brisker taste than when taken out of a 

 glass. Professor Robison ascribed this 

 to the influence of galvanism, arising 

 from the circle formed by the metal and 

 two different .'fluids. He considered 

 that in the act of drinking, one side of 



the pewter pot is exposed to the action 

 of the saliva which moistens the lip, 

 while the other side of the metal is 

 touched by the porter ; the circle being 

 completed when the latter fluid comes 

 in contact with the tongue. 



(81.) The second kind of galvanic 

 combinations with a single metal, con- 

 sists of a series of plates composed of a 

 metal capable of being acted upon by 

 sulphuretted hydrogen, in contact with 

 solutions of hydrosulphurets on the one 

 side, and water on the other, placed in 

 a regular order of alternation. Under 

 these circumstances, a current of elec- 

 tricity is produced, the direction of 

 which is the reverse of what it is in the 

 former case, the surface of the metallic 

 plate in contact with the solution of sul- 

 phur being positive, while that in con- 

 tact with acid is negative. Eight series 

 will produce sensible effects. Copper, 

 silver, and lead are each capable of 

 forming this combination; their com- 

 parative activity being in the order in 

 which they are here enumerated, that 

 is, copper the most, and lead the least.* 



(82.) A familiar instance of the ope- 

 ration of galvanism in promoting the 

 combination of sulphur with silver, 

 occurs in the employment of a silver 

 spoon in eating the yolk of an egg ; a 

 galvanic circle of the second kind being 

 formed by the yolk, which contains 

 sulphur, the silver spoon, and the saliva 

 of the tongue. 



(83.) The third kind of combinations 

 unite the power of the two former, and 

 consist of a single metal, acted upon on 

 one side by an acid, and on the other 

 side by the hydro- sulphurets. Copper, 

 silver, or lead may here be employed, 

 and the order of their powers is the 

 same as in the preceding instance. The 

 pile may be constructed in the same 

 manner as the pile with zinc in the first 

 kind of combination ; the cloths moist- 

 ened with acid being separated from 

 those moistened with solution of hydro- 

 sulphuret by an intermediate cloth 

 soaked in solution of sulphate of pot- 

 ash. Three plates of copper, or silver, 

 arranged in this manner, in proper 

 order, produce sensible effects ; and a 

 pile composed of twelve or thirteen 

 series is capable of giving weak shocks 

 and of rapidly decomposing water. The 

 current of electricity is determined as in 

 the two former cases. 



(84.) Greater permanency may be 



* Philosophical Transactions for 1801, p. 400. 



