358 VHILOSOl'HICAL TRANSACTIONS. [ANNO 1783. 



adding the common air, be .08 of a measure. Let us now suppose, that the 

 quantity of common air used is 1.02 of a measure, then will the diminution on 

 adding the common air, be 1.08 X ~ or 1.1010 of a measure, and conse- 

 quently the increase of weight of the bottle m will be 1.1016 — 1.02 or .O8l6of 

 a measure, which is very nearly the same as if the common air used had been 

 exactly one measure. 



In the 2d method of proceeding, or that in which the nitrous air is added to 

 the respirable, I use always the same bottle, namely, that which holds 3 mea- 

 sures, and v e always 1 measure of respirable air ; and in trying common air use 

 the same vessel a as in the first method ; but for dephlogisticated air I use one 

 that holds 3f measures. In trying the experiment, I first weigh the bottle m 

 without any air in it, and then weigh it again with the respirable air in it, which 

 gives the quantity of respirable air used. I next put the nitrous air into the 

 vessel a, and weigh that and the bottle m together, and then having mixed the 

 airs, weigh them again, which gives the diminution. 



From what has been just said, it appears, that in this method of proceeding I 

 use a less quantity of nitrous air in trying the same kind of respirable air than in 

 the former; the reason of which is, that the same quantity of nitrous air goes 

 further in phlogisticating a given quantity of respirable air in this than in the 

 former method. In both these methods, I express the test of the air by the 

 diminution which they suffer in mixing ; for example, if the diminution on 

 mixing them be 2 measures and -j^V* I call its test 2.353, and so on. 



There is a considerable difference in the diminution, according to the nature of 

 the water, which is a very great inconvenience, and seems to be the chief cause 

 of uncertainty in trying the purity of air ; but it is by no means peculiar to this 

 method, as I have found as great a difference in Foil tana's method, according as 

 I have filled the tube with different waters. But it shows plainly, how little all 

 the experiments which have hitherto been made, for determining the variations 

 in the purity of the atmosphere, can be relied on, as I do not know that any 

 one before has been attentive to the nature of the water he has used, and the 

 difference proceeding from the difference of waters is much greater than any I 

 have yet found in the purity of air. The best way I know of obviating this in- 

 convenience, is to be careful always to use the same kind of water: that which 

 I always use is distilled, as being most certain to be always alike. I should have 

 used rain water, as being easier procured, if it had not been that this water is 

 sometimes apt to froth, which I have never known distilled water do. 



Mr. C. adds several other cautionary observations, which occurred in his prac- 

 tice in making experiments with his eudiometer; and then proceeds as follows. 



During the last half of the year 1781, I tried the air of near 60 different 

 days, in order to find whether it was sensibly more phlogisticated at one time 



