INSTANCES OF DISCOVERY OF RESIDUAL PHENOMENA. 433 



engaged from it, so that the water would have no share in 

 iheir production. Assuming this, he proceeded to try 

 whether the total removal of the cause would destroy the 

 affect, or at least the diminution of it cause a corresponding 

 change in the amount of effect produced. By the sub- 

 stitution of gold vessels for the glass without any change 

 n the effect, he at once determined that the glass was not 

 ,he cause. Employing distilled water, he found a marked 

 liminution of the quantity of acid and alkali evolved ; yet 

 ;here was enough to show that the] cause, whatever it was, 

 was still in operation. The impurity of the water then was 

 not the sole, but a concurrent cause. He now conceived 

 that the perspiration from the hands touching the instru- 

 ments might affect the case, as it would contain common 

 $alt, and an acid and an alkali would result from its 

 decomposition under the agency of electricity. By care- 

 'ully avoiding such contact, he reduced the quantity of the 

 )roducts still further, until no more than slight traces of 

 ihem were perceptible. What remained of the effect 

 might be traceable to impurities of the atmosphere, de- 

 3omposed by contact with the electrical apparatus. An 

 experiment determined this ; the machine was placed 

 inder an exhauster receiver, and when thus secured from 

 itmospheric influence, it no longer evolved the acid and 

 ilkali. 1 



1 Archbishop Thomson, Outline of the Lams of Thought, p. 225. See 

 ilso Chap. XXXVI. p. 356. 



Respecting the same experiments, Dr. Thomson, in his History of 

 Chemistry , p. 260, says : ' It was his celebrated paper on some Chemical 

 Agencies of Electricity, inserted in the Philosophical Transactions for 

 L807, that laid the foundation of the high reputation which he so 

 leservedly acquired. I consider this paper not merely as the best of 

 all his own productions, but as the finest and completest specimen of 

 inductive reasoning which appeared during the age in which he lived. 

 [t had been already observed, that when two platinum wires from the 



