i26 Mr . cavendish’s x'lccount of 
when the tefts tried with the air from iron came out fmaller 
than the reft by not lefs than ^.y^ths. It Ihould feem, there- 
fore, from thefe experiments, that the nitrous air procured 
from iron, bcfides being much more impure than the others, 
differs from them alfo in the fecond refpedt ; that is, that the 
pure nitrous air in it contains rather lefs phlogifton than that 
in the others : whence it happens, that a greater quantity is 
neceffary to phlogifticate a given portion of common air, and 
conlequently that the diminution is greater when a fufficient 
quantity of it is ufed, though with a lefs proportion the dimi- 
nution is much lefs than with other nitrous air, on account of 
its greater impurity. As for the air procured from the three 
other fubftances, I cannot be lure that there is any difference 
between them. The nitrous air I always ufe is made from 
copper, as it is procured with lefs trouble than from quick- 
filver, and I have no reafon to think it more likely to vary in 
its quality. 
During the laft half of the year 1781, I tried the air of 
near 60 different days, in order to find whether it was fenfibly 
more phlogifticated at one time than another ; but found no 
difference that I could be fure of, though the wind and wea- 
ther on thofe days were very various ; fome of them being 
very fair and clear, others very wet, and others very foggy. 
My way was to fill bottles with glafs ftoppers every now and 
then with air from without doors, and preferve them flopped 
and inverted into water, till I had got feven or eight, and then 
take their teft ; and whenever I obferved their teft, I filled two 
bottles, one of which was tried that day, and the other was kept 
till the next time of trying, in order to fee how nearly the teft of 
the fame air, tried on different days, would agree. The expe- 
riment was always made with diftilled water, and care was 
7 always 
