1 12 Mr. cavendish's Account of 
lei's than one meafure, that is, than the refpirable air alone, it 
is a fign that the quantity of nitrous air is f efficient, or that it 
is fufficient to produce the full diminution, unlefs it is very 
impure. 
Though the quantity of refpirable air ufed will be always 
nearly the fame, as being put in by meafure ; yet it will com- 
monly be not exactly fo, for which reafon the obferved dimi- 
nution will commonly require fome corredtion : for example, 
l'uppofe that the oblerved diminution was 2.353 meafures, and 
that the quantity of refpirable air was found to be .9 li 5 of a 
meafure ; then the obferved diminution muff be increafed by 
of the whole or .035, in order to have the true dimi- 
nution, or that which would have been produced if the refpi- 
rable air ufed had been exactly one meafure ; confequently, the 
true diminution is 2.588. 
The method of weighing, defcribed in p. 109. is that which I 
ufe in trying air much different in purity from common air ; 
but in trying common air, I ufe a fhorter method, namely, I 
do net weigh the veffel A at all, but only weigh the bottle M 
with the nitrous air in it ; then mix the airs, and again weigh 
the fame bottle with the mixture in it, and find the increafe of 
weight. This, added to one meafure, is very nearly the true 
diminution, whether the quantity of common air ufed was a 
little more or a little lefs than one meafure. The reafon of 
this is, that as the diminution produced on mixing common 
and nitrous air is only a little greater than the bulk of the com- 
mon air, the bulk of the mixture will be very nearly the fame, 
whether the bulk of the common air is a little greater or a little 
lels than one meafure : for example, let us firft fuppofe, that 
the quantity of common air ufed is exactly one meafure, and 
that the diminution of bulk on mixing is 1.08 of a meafure, 
then 
