Chemical Decomposition by common Electricity, 255 



colour visible, and allows of the point of contact between it 

 and the decomposing wires being contracted to the utmost de- 

 gree. A piece of paper moistened in the solution of iodide of 

 potassium and starch, or of the iodide alone, with certain pre- 

 cautions (322.), is a most admirable test of electro-chemical 

 action ; and when thus placed and acted upon by the electric 

 current, will show iodine evolved at p by only half a turn of 

 the machine. With these adjustments and the use of iodide 

 of potassium on paper, chemical action is sometimes a more 

 delicate test of electrical currents than the galvanometer 

 (273.). Such cases occur when the bodies traversed by the 

 current are bad conductors, or when the quantity of electricity 

 evolved or transmitted in a given time is very small. 



317. A piece of litmus paper, moistened in solution of com- 

 mon salt or sulphate of soda was quickly reddened at p, A 

 similar piece moistened in muriatic acid was very soon bleach- 

 ed at p. No effects of a similar kind took place at n. 



318. A piece of turmeric paper, moistened in solution of 

 sulphate of soda, was reddened at n by two or three turns of 

 the machine, and in twenty or thirty turns plenty of alkali 

 was there evolved. On turning the paper round, so that the 

 spot came under p, and then working the machine, the alkali 

 soon disappeared, the place became yellow, and a brown alka- 

 line spot appeared in the new part under n. 



319. On combining a piece of litmus with a piece of tur- 

 meric paper, wetting both with solution of sulphate of soda, 

 and putting the paper on the glass, so that p was on the litmus 

 and n on the turmeric, a very few turns of the machine suf- 

 ficed to show the evolution of acid at the former and alkali 

 at the latter, exactly in the manner effected by a volta- 

 electric current. 



320. All these decompositions took place equally well, 

 whether the electricity passed from the machine to the foil a 9 

 through water, or through wire only ; by contact with the con- 

 ductor, or by sparks there ; provided the sparks were not so 

 large as to cause the electricity to pass in sparks from p to n 9 

 or towards n; and I have seen no reason to believe that in 

 cases of true electro-chemical decomposition by the machine, 

 the electricity passed in sparks from the conductor, or at any 

 part of the current, is able to do more, because of its tension, 

 than that which is made to pass merely as a regular current. 



321. Finally, the experiment was extended into the follow- 

 ing form, supplying in this case the fullest analogy between 

 common and voltaic electricity. Three compound pieces of 

 litmus and turmeric paper (319.) were moistened in solution 

 of sulphate of soda, and arranged on a plate of glass with 



