Chemical Effects of common Electricity, 257 



of the current, nor any increase of intensity allowed by which 

 the electricity may be induced to pass between the platina 

 wires and the moistened papers, otherwise than by conduc- 

 tion ; for if it burst through the air, the effect referred to 

 ensues. 



324. The effect itself is due to the formation of nitric acid 

 by the combination of the oxygen and nitrogen of the air, and 

 is, in fact, only a delicate repetition of Cavendish's beautiful 

 experiment. The acid so formed, though small in quantity, 

 is in a high state of concentration as to water, and produces 

 the consequent effects of reddening the litmus paper, or pre- 

 venting the exhibition of alkali on the turmeric paper, or, by 

 acting on the iodide of potassium, evolving iodine. 



325. By moistening a very small slip of litmus paper in so- 

 lution of caustic potassa, and then passing the electric spark 

 over its length in the air, I gradually neutralized the alkali, 

 and ultimately rendered the paper red ; on drying it, I found 

 that nitrate of potassa had resulted from the operation, and 

 that the paper had become touch-paper. 



326. Either litmus paper or white paper moistened in so- 

 lution of iodide of potassium, offers therefore a very simple, 

 beautiful, and ready means of illustrating Cavendish's experi- 

 ment of the formation of nitric acid from the atmosphere. 



327. I have already had occasion to refer to an experi- 

 ment (265. 309.) by Dr.Wollaston, which is insisted upon too 

 much, both by those who oppose and those who agree with 

 the accuracy of his views respecting the identity of voltaic and 

 ordinary electricity. By covering fine wires with glass or 

 other insulating substances, and then removing only so much 

 matter as to expose the point, or a section of the wires, and 

 by passing electricity through two such wires, the guarded 

 points of which were immersed in water, Wollaston found 

 that the water could be decomposed even by the current from 

 the machine, without sparks, and that two streams of gas 

 arose from the points, exactly resembling in appearance, those 

 produced by voltaic electricity, and, like the latter, giving a 

 mixture of oxygen and hydrogen gases. But Dr. Wollaston 

 himself points out that the effect is different from that of the 

 voltaic pile, inasmuch as both oxygen and hydrogen are 

 evolved from each pole ; he calls it " a very close imitation of 

 the galvanic phenomena," but adds, that "in fact the resem- 

 blance is not complete," and does not trust to it to establish 

 the principles, correctly laid down in his paper. 



328. This experiment is neither more nor less than a repe- 

 tition, in a refined manner, of that made by Dr. Pearson in 



Third Series, Vol. 3. No. 16. Oct, 1833. 2 L 



