122 Mr. cavendish’s Account of 
eating common air in fome circumftances than others, which 
is the caufe that the diminution in trying the purity of air by 
the nitrous teft is fo much greater in fome methods of mixing 
them than in others. 
From what was faid in p. i 19. it fhould feem as if the fe- 
cond method was more exadt than the firft, as the error pro- 
ceeding from the air employing more or lels time in palling 
into the bottle was found to be lefs, and that proceeding from a 
difference in the water, and from the bottle being fhaken more 
or lefs ftrongly was not greater. I, however, have found, that 
the trials of the fame air on the fame day have commonly dif- 
fered more when made in this manner than in the firft ; for 
which reafon, and becaufe in trying common air the firft me- 
thod takes up the leaft time, I have commonly ufed that. 
It fhould be obferved, that in trying dephlogifticated air by 
the firft method it is convenient to ule different bottles, accord- 
ing to the different purity of the air ; and the fame air will 
appear purer, if tried by a larger bottle than by a fmaller. For 
example, if its teft, tried by the large bottle, comes out 2.54, 
it will appear not more than 2.44, if tried by the middle bot- 
tle ; and, in like manner, if its teft by the middle bottle comes 
out 1. 1 1, it will appear to be about I.c 8 , if tried by the leaft 
bottle ; for this reafon it is right always to fet down which 
bottle it is tried by. 
I think I may confidently aflert, that either of the above 
methods are confiderably more accurate than Fontana’s, fup- 
pofing the experiment to be made exaflly in his manner, that 
rs; determining the quantities by meafure. But, in order to 
judge which method of mixing the airs is moft exaff, it was ne- 
ceflary to determine the quantities in his method alfo by weight, 
as otherwife it would be uncertain whether my method of 
mixing 
