a new Eudiometer. 129 
year in bottles holding about a pint, which I have no reafon to 
think was at all injured; but then I have tried fome kept not 
more than one-third part of that time which Teemed to have 
been a little impaired, though I do not know what it could be 
owing to, unlefs it was that the bottles were fmaller, namely, 
holding lefs than one-fourth of a pint, and that in all of them, 
except two, which were fmaller than the reft, the ftopper which, 
however, fitted in very tight, was tied down by a piece of 
bladder. 
I made fome experiments aifo to try whether the air was fen- 
fibly more dephlogifticated at one time of the day than another, 
but could not find any difference. I alfo made feveral trials with 
a view to examine whether there was any difference between the 
air of London and the country, bv filling bottles with air on the 
fame day, and nearly at the fame hour, at Marlborough-ftreet 
and at Kensington. The refult was, that fometimes the air of 
London appeared rather the pureft, and fometimes that of Ken- 
sington ; but the difference was never more than might pro- 
ceed from the error of the experiment ; and by taking a mean 
of all, there did not appear to be any difference between them. 
The number of 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. 
It is very much to be wifhed, that thofe gentlemen who 
make experiments on factitious airs, and have occafion to ascer- 
tain their purity by the nitrous teft, would reduce their obfer- 
vations to one common fcale, as the different inftruments em- 
ployed for that purpofe differ So much, that at prefent it is 
almoft impoftible to compare the obfervations of one perfon 
with thofe of another. This may be done, as there feems to 
be fo very little difference in the purity of common air at dift- 
Vol. LXXIII. S ferent 
