1C 



GALVANISM. 



and subjected to the action of the vol- 

 taic battery. If the liquid contain any 

 soluble saline compound, such as sul- 

 phate of soda, or common Glauber's 

 salt, and the operation be continued a 

 sufficient time, the whole of the acid 

 contained in the salt will be found col- 

 lected in the positive cup, and the whole 

 of the alkali in the negative cup. Nor 

 is any considerable solubility in the body 

 placed in the circuit necessary for its 

 decomposition by galvanism. Two cups 

 made of compact sulphate of lime, con- 

 taining pure water, were connected to- 

 gether by fibrous sulphate of lime, moist- 

 ened by pure water, and the voltaic 

 current transmitted through them, Af- 

 ter an hour the fluids were accurately 

 examined, when it was found that the 

 negative cup contained a pure and satu- 

 rated solution of lime, partially covered 

 with a calcareous crast ; while the po- 

 sitive cup was filled with a moderately 

 strong solution of sulphuric acid. Sul- 

 phate of strontites, and fluate of lime, 

 subjected to the same process, yielded 

 similar results : sulphate of barytes, 

 from its greater insolubility, proved 

 more difficult of decomposition ; but the 

 difficulty was at length overcome. The 

 analysis of many mineralogical speci- 

 mens, 'of which the composition was 

 much more complicated, was greatly 

 elucidated by the application of voltaic 

 electricity, which effected the extraction 

 of all the acid and alkaline matters they 

 contained. 



(48.) For the production of these 

 effects it is immaterial in what part of 

 the fluid line of circuit the decomposable 

 body happens to be situated. This will 

 appear by placing three cups, side by 

 side, in a line (fig. 19.), and connecting 



Fig. 19. 



them together by moistened asbestos. 

 Let a solution of sulphate of potash, or 

 any other neutral salt, be put into the 

 middle cup, and blue infusion of cab- 

 bage into the other cups. When these 

 fluids are placed in the circuit of the 

 voltaic battery, by immersing the wires 

 into the fluid in the outer cups, the sul- 

 phuric, or other acid, will collect in the 



positive cup, and render its blue infu* 

 sion red, while the alkali will pass into 

 the opposite cup, and tinge its blue con- 

 tents green. 



(49.) When metallic solutions are 

 subjected to the decomposing action of 

 galvanism, a deposition of the metal, 

 generally in the form of minute crystals, 

 takes place on the negative wire, and 

 oxide is also deposited around it ; while 

 the acid passes over, as before, into the 

 positive cup. This effect takes place 

 with solutions of iron, zinc, and tin, as 

 well as with the more oxidable metals. 



(50.) When a solution of nitrate of silver 

 has been placed on the positive side, and 

 distilled water on the negative, the whole 

 of the connecting asbestos becomes co- 

 vered with a thin metallic film of silver. 

 We have been the more particular in 

 noticing these effects, because, as was 

 before observed ($ 41.), they occur to a 

 greater or less extent in the fluids which 

 occupy the cells of the battery, and have 

 a considerable influence in modifying, 

 and ultimately destroying the power of 

 the instrument. 



(51.) Phenomena of a still more ex- 

 traordinary nature, presented themselves 

 to Sir H. Davy in the further prosecu- 

 tion of these inquiries. It was disco- 

 vered that the elements of compound 

 bodies were actually conveyed by the 

 influence of the electric current through 

 solutions of substances, on which, under 

 other circumstances, they would have 

 exerted an immediate and powerful che- 

 mical action, without any such effect 

 being produced. Acids, for example, 

 may be transmitted from one cup, con- 

 nected with the negative pole, to ano- 

 ther cup on the opposite or positive 

 side, through a portion of fluid in an 

 intermediate cup tinged with any of the 

 vegetable coloured infusions, which are 

 instantly reddened by the presence of 

 an acid, without occasioning the slight- 

 est change of colour. The same hap- 

 pens also with alkalies. If three cups 

 be arranged as before, (see Jig. 19.) and 

 connected with each other in a series by 

 moistened cotton, the middle cup, and 

 also the one next to the positive side of 

 the battery, being filled with blue in- 

 fusion of cabbage, or of litmus ; and the 

 cup next to the negative side containing 

 a solution of sulphate of soda ; on the 

 series being placed in the voltaic circuit, 

 a red tinge will soon be perceived in 

 the water of the positive cup, which 

 will become strongly acid. It is evi- 

 dent that the sulphuric acid so trans- 



