238 ELEMENTARY LESSONS ON [CHAP. iv. 



points is mentioned in Art. 43, and the mechanical 

 effects of a current of electricity were described in 

 Lesson XIX. 



285. Lullin's Experiment. If a piece of card- 

 board be perforated by a spark between two metal points, 

 two curious facts are observed. Firstly, there is a slight 

 burr raised on each side, as if the hole had been pierced 

 from the middle outwards. Secondly, if the two points 

 are not exactly opposite one another the hole is found 

 to be nearer the negative point. But if the experiment 

 is tried under the air pump in a vacuum, there is no 

 such displacement of the hole ; it is then midway 

 exactly. 



286. Chemical Effects. The chemical actions 

 produced by currents of electricity have been described 

 in Lessons XIV. and XVIII. Similar actions can be 

 produced by the electric spark, and by the silent glow 

 discharge (see Art. 290). Faraday showed, indeed, that 

 all kinds of electricity from different sources produced the 

 same kinds of chemical actions, and he relied upon this 

 as one proof of the essential identity of the electricity 

 produced in different ways. If sparks from an electric 

 machine are received upon a piece of white blotting- 

 paper moistened with a solution of iodide of potassium, 

 brown patches are noticed where the spark has effected 

 a chemical decomposition and liberated the iodine. 



When a stream of sparks is passed through moist air 

 in a vessel, the air is found to have acquired the property 

 of changing to a red colour a piece of paper stained 

 blue with litmus paper. This, Cavendish showed, was 

 due to the presence of nitric acid, produced by the 

 chemical union of the nitrogen and oxygen of the air. 

 The effect is best shown with the stream of sparks 

 yielded by a small induction coil (Fig. 148), in a vessel 

 in which the air has been compressed beyond the usual 

 atmospheric pressure. 



The spark will decompose ammonia gas, and olefiant 



