and its Effects. 123 



weak, because the sparks are small and have little intensity of 

 light. It is more perfect with weak sulphuric acid, hydrochloric 

 acid, nitric acid, with solutions of hydrate of potash, carbonate 

 of soda, &c. ; with these alkaline liquids the middle part of the 

 stream of sparks has a beautiful yellow colour, at the positive 

 pole it is even gold-yellow. The phsenomenon, however, is most 

 perfect with concentrated sulphuric acid : not one of the liquids 

 which I examined showed the blue disc on its surface under- 

 neath the positive pole so great and clear, and beautifully 

 coloured. All the above-mentioned phsenomena are more fully 

 developed when the experiments are made in air rarefied to about 

 8 inches of mercury pressure. 



No chemical decomposition can be seen in any of the above 

 cases when both poles send sparks to the liquid. If, however, 

 sparks are allowed to issue from one wire only, and the other is 

 held immersed, a decomposition is immediately visible at the 

 latter, and it is quite indifferent which pole is immersed, and 

 from which sparks are emitted. This remarkable decomposi- 

 tion at a single actual electrode is certainly weak, but I have 

 distinctly observed it with dilute acids, and still more distinctly 

 with concentrated sulphuric acid. 



With a solution of iodide of potassium the action is different, 

 for it is decomposed even when sparks fall upon it from both 

 poles ; consequently when there are no real electrodes. Whether 

 nitric acid or ozone is thereby formed, may remain at present 

 undiscussed. 



If a piece of blotting-paper be moistened with this solution, 

 put in contact with the negative pole, and the positive pole 

 be brought sufficiently near, a brown spot will immediately 

 make its appearance underneath the latter, and the positive 

 stream of sparks will have a brownish colour unlike the violet 

 colour of the vapour of iodine. Underneath the negative pole 

 no colour is visible. But if, on the contrary, the paper be 

 touched by the positive pole and receive sparks from the nega- 

 tive, a brown colour will be perceptible underneath both, though 

 much weaker underneath the latter. Something similar is 

 observed when the negative pole is connected with the earth, 

 and a piece of moist iodide of potassium paper is held opposite 

 the positive pole only. 



Paper and sparks have then a brown colour. If sparks are 

 allowed to strike upon the paper from both poles, a brownish 

 colour will generally be perceptible underneath both, but always 

 weaker under the negative than under the positive. 



If the two moist places upon the paper which are struck by 

 the sparks be separated by a dry part of small breadth, sparks 

 will also spring over the latter. 



K. 2 



