VOL. LXXIII.] PHILOSOPHICAL TRANSACTIONS. 357 



indeed if it were much greater, it would be of very little consequence, as it 

 would be always the same in trying the same kind of air. 



The vessel a holds 282 grains of water, and is the quantity which I shall dis- 

 tinguish by the name of ] measure. I have 3 bottles for mixing the airs in, with 

 a measure b for the nitrous air adapted to each. The first bottle holds 3 mea- 

 sures, and the corresponding measure 1-J-; the 2d bottle holds 6 ; and the cor- 

 responding measure 2-1- ; and the 3d bottle holds 12, and the corresponding 

 measure 5. The first bottle and measure is used in trying common air, or air 

 not better than that ; the other two in trying dephlogisticated air. The quan- 

 tity of respirable air used, as before said, is always the same, namely, 1 measure; 

 consequently, in trying common air, I use 1-J- measure of nitrous air to 1 of 

 common ; and in trying very pure dephlogisticated air, I use 5 measures of 

 nitrous air to 1 of the dephlogisticated. I believe there is no air so much dephlo- 

 gisticated as to require a greater proportion of nitrous than that. The way by 

 which I judge whether the quantity of nitrous air used is sufficient, is by the 

 bulk of the two airs when mixed, for if that is not less than one measure, that 

 is, than the respirable air alone, it is a sign that the quantity of nitrous air is suffi- 

 cient, or that it is sufficient to produce the full diminution, unless it be very impure. 



Though the quantity of respirable air used will be always nearly the same, as 

 being put in by measure; yet it will commonly be not exactly so, for which 

 reason the observed diminution will commonly require some correction ; for 

 example, suppose that the observed diminution was 2.353 measures, and that 

 the quantity of respirable air was found to be .985 of a measure; then the 

 observed diminution must be increased by -j-fjh- of the whole or .035, in order 

 to have the true diminution, or that which would have been produced if the 

 respirable air used had been exactly 1 measure ; consequently the true diminution 

 is 2.388. 



The method of weighing, before described, is that which I use in trying air 

 much different in purity from common air ; but in trying common air, I use a 

 shorter method, namely, I do not weigh the vessel a at all, but only 

 weigh the bottle m with the nitrous air in it ; then mix the airs, and again weigh 

 the same bottle with the mixture in it, and find the increase of weight. This, added 

 to 1 measure, is very nearly the true diminution, whether the quantity of com- 

 mon air used was a little more or a little less than 1 measure. The reason of this is, 

 that as the diminution produced on mixing common and nitrous air is only a 

 little greater than the bulk of the common air, the bulk of the mixture will be 

 very nearly the same, whether the bulk of the common air is a little greater or a 

 little less than 1 measure : for example, let us first suppose, that the quantity of 

 common air used is exactly 1 measure, and that the diminution of bulk on mix- 

 ing is 1.08 of a measure, then must the increase of weight of the bottle m, on 



