202 
naked eye ; and the negative wire, being afterwards 
placed on the same spot, soon restored to the card 
its original blue colour. Similar results were ob- 
tained, in less time, by the different electricities of 
the Voltaic battery *. 
47-5. Now, in all the foregoing examples, there is 
just reason to conclude, that the changes of colour 
were, in every instance, produced by the formation, 
or development of acid or alkaline matter : and 
the sources from which both might be derived are 
clearly pointed out in the late refined analyses of Mr 
Davy. He found that water, after repeated distilla- 
tion, still contained saline matter, which was decom- 
posable by electrization ; that this matter was like- 
wise obtained from the substance of glass, and from 
every earthy body ; and, lastly, that the nitrogen, 
naturally existing in water, by combining with nas- 
cent oxygen or hydrogen derived from its decompo- 
sition, formed, respectively, nitric acid or ammonia, 
which were easily discoverable by the proper tests. 
But when the water which he used was rendered 
perfectly pure ; when no saline matter existed in 
the substance of the vessels employed ; and when 
nitrogen was wholly excluded, by conducting the 
operation in vessels of pure hydrogen gas, then 
neither acid nor alkaline matter was either generated 
or developed, but the water was resolved into its 
constituent gases alone ! 
466. From these facts, we learn why, in Dr 
Priestley's experiment (474.), air seemed necessary 
Phil, Trans. 1 801, p, 432 f t Ibid. 1807- 
