504 PHILOSOPHICAL TRANSACTIONS. [aNNO 1774. 



examining it, found it to be good common air; at least a candle burned in it very 

 well. I had not then discovered the method of ascertaining the goodness of 

 common air, by a mixture of nitrous air. Previous to the trial, I had suspected 

 that this air would have been found to be inflammable, 



I shall conclude this letter with observing, that I have found a remarkable 

 difference in different kinds of water, with respect to their effect on common air 

 agitated in them, and which I am not yet able to account for. If I agitate 

 common air in the water of a deep well, near my house in Calne, which is hard, 

 but clear and sweet, a candle will not burn in it after 3 minutes. The same is 

 the case with the rain water which I get from the roof of my house. But in 

 distilled water, or the water of a spring well near the liouse, I must agitate the 

 air about 20 minutes, before it will be so much injured. It may be worth while 

 to make further experiments, with respect to this property of water. 



In consequence of using the rain water, and the well water abovementioned, 

 I was very near concluding, contrary to what I have asserted in my printed 

 papers, that common air suffers a decomposition by great rarefaction. For when I 

 had collected a considerable quantity of air, which had been rarefied about 400 

 times, by an excellent pump made forme by Mr, Smeaton, I always foun(^l, that when 

 I filled my receivers with the water above-mentioned, though I did it so gradually 

 as to occasion as little agitation as possible, a candle would not burn in the air 

 that remained in them. But .when I used distilled water, or fresh spring water, 

 I undeceived myself. 



" p. s. I cannot help expressing my surprize, that so clear and intelligible an 

 account, of Mr. Smeaton's air pump, should have been before the public so 

 long, as ever since the publication of the 47th vol. of the Philos. Trans., and 

 yet that none of our philosophical instrument makers should attempt the con- 

 struction. The superiority of this pump, to any that are made on the common 

 plan, is indeed prodigious. Few of them will rarefy more than 100 times, and 

 in a general way not more than 6o or 70 times; whereas this instrument must 

 be in a poor state indeed, if it do not rarefy 200 or 300 times; and when in 

 good order, it will go as far as 1 000 times, and sometimes even much farther 

 than that; besides, this instrument is worked with much more ease, than a 

 common air pump, and either exhausts or condenses at pleasure. In short, to a 

 person engaged in philosophical pursuits, this instrument is an invaluable acquisi- 

 tion, 1 shall have occasion to recite some experiments, which I could not have 

 made, and which indeed I should hardly have dared to attempt, if I had not been 

 possessed of such an air pump as this. It is much to be wished, that some 

 person of spirit in the trade would attempt the construction of an instrument, 

 which would do great credit to himself, as well as be of eminent service to 

 philosophy." 



