360 PHILOSOPHICAL TRANSACTIONS. [ANNO 1783. 



days compared was 20, and a great part of them taken in winter, when there are 

 a greater number of fires, and on days when there was very little wind to blow 

 away the smoke. 



Jt is very much to be wished, that those gentlemen who make experiments on 

 factitious airs, and have occasion to ascertain their purity by the nitrous test, 

 would reduce their observations to one common scale; as the different instruments 

 employed for that purpose differ so much, that at present it is almost impossible 

 to compare the observations of one person with those of another. This may be 

 done, as there seems to be so very little difference in the purity of common air 

 at different times and places, by assuming common air and perfectly phlogisti- 

 cated air as fixed points. Thus, if the test of any air be found to be the same 

 as that of a mixture of equal parts of common and phlogisticated air, I would 

 say, that it was half as good as common air: or, for shortness, I would say, 

 that its standard was i: and, in general, if its test was the same as that of a mix- 

 ture of one part of common air and x of phlogisticated air, I would say, that its 

 standard was — — . In like manner, if one part of this air would bear being 

 mixed with x of phlogisticated air, in order to make its test the same as that of 

 common air, I would say, that it was 1 -f- x times as good as common air, or 

 that its standard was 1 + x 't consequently, if common air, as Mr. Scheele and 

 Lavoisier suppose, consist of a mixture of dephlogisticated and phlogisticated 

 air, the standard of any air is in proportion to the quantity of pure dephlogisti- 

 cated air in it. In order to find what test on the eudiometer answers to different 

 standards below that of common air, all that is wanted is, to mix common and 

 perfectly phlogisticated air in different proportions, and to take the test of those 

 mixtures; but in standards above that of common air, it is necessary to procure 

 some good dephlogisticated air, and to find its standard by trying what propor- 

 tion of phlogisticated air it must be mixed with, in order to have the same test 

 as common air, and then to mix this dephlogisticated air with different propor- 

 tions of phlogisticated air, and find the test of those mixtures.* 



Perfectly phlogisticated air may be conveniently procured by putting some so- 

 lution of liver of sulphur into a bottle of air well stopped, and shaking it fre- 

 quently till the air is no longer diminished, which, unless it is shaken very fre- 

 quently, will take up some days. Care must be taken however, to loosen the 



* The rule for computing the standard of any mixture of dephlogisticated and phlogisticated air is 

 as follows. Suppose that the test of a mixture of D parts of dephlogisticated air with r of phlogis- 

 ticated air, is the same as that of common air; then is the standard of the dephlogisticated air 



. Let now ^ parts of this dephlogisticated air be mixed with <p parts of phlogisticated air, the 



i) + p $ 

 standard of the mixture will be x t -„. — Orig. 



D <? + <P 



