434 Prof. Andrews on the Thermo-electric Currents 



circuit, the connecting poles in the water being formed of 

 platina wires. On fusing the globule as before, the needle of 

 the galvanometer was still deflected through an arc of 80° or 

 90°, but with less violence than when a complete metallic cir- 

 cuit was employed. When carbonate of soda was substituted 

 for borax in these experiments, similar but more powerful 

 currents were obtained. 



My first attempts to obtain chemical decompositions by 

 means of these currents were unsuccessful when the common 

 forms of apparatus were used; but by employing poles ex- 

 posing unequal surfaces, this object was finally attained*. A 

 piece of bibulous paper, exposing on each side a surface of 

 one fourth of a square inch, was moistened with a solution of 



* The influence of the surface of the poles, in rendering perceptible the 

 separation of the elements of an electrolyte, is very remarkable. Faraday 

 has observed that not a bubble of gas will appear on the surface of a pair 

 of platina plates, immersed in dilute sulphuric acid, when made the poles 

 of a voltaic combination, formed liy a single pair of platina and zinc plates 

 charged with the same dilute acid; and hence that distinguished philosopher 

 has inferred, that the tension of such a current is too low to effect the de- 

 composition o'i water. On repeating and varying the conditions of this 

 experiment, I found that if two fine wires were substituted for the platina 

 plates the same negative result was obtained ; but that if a platina plate 

 exposing an extensive surface to the liquid was used as one pole, and a fine 

 wire of the uame metal as the other, then a minute stream of bubbles of 

 gas arose from the wire, which after continuing for some time finally ceased 

 to appear. An additional quantity of gas was, however, easily procured, 

 either by increasing the surface of the broad pole, or by removing it and 

 heating it to redness, or by reversing the direction of the current. The 

 following appears to be a satisfactory explanation of these results. When 

 the poles exposed on both sides equal surfaces, the gases were dissolved 

 in the nascent state by the surrounding liquid ; but when the polar sur- 

 faces were unequal, the solution of the gas being greatly facilitated by the 

 broader pole, the element Ki'i the water separated there was dissolved, 

 while the other element was disengaged, in the gaseous state, at the wire 

 which served as the opposite pole. Indeed, Becquerel had already cor- 

 rectly inferred, from the circumstance of the plates acquiring polarity, that 

 the water in this experiment of Faraday must have been decomposed. It 

 is frou) the obstacle presented to the passage of the current by the acquired 

 polarity of the platina plate, that the gas soon ceases to be formed in 

 greater abundance than it can be dissolved by the water; and its reap- 

 pearance under the circumstances stated liefore, is an obvious consequence 

 from the well-known properties of polarized plates. By employing a si- 

 milar artifice, a solution of suljjhate of soda may be decomposed by means 

 of a single couple of platina and zinc plates, charged with a solution of chlo- 

 ride of sodium, and the presence of the free acid or alkali rendered evident 

 by its action on litnms or turmeric paper. In order therefore to discover, in 

 case of difficulty, whether an electrical current is capable of decomposing 

 water or other substances, it is necessary to employ poles having very un- 

 equal surfaces ; and this will be effected in the most perfect manner by 

 opposing a thick wire or plate of platina to one of Wollaston s guarded 

 points. 



