[ 93 ] 
of danger, in a fituation in which they tnuft ne- 
ceflarily breathe it. On this account, I hav* 
thought it right to make this communication as 
early as I conveniently could ; and as Dr. Alex- 
ander appears to be an ingenuous and benevolent 
man, I doubt not but he will thank me for it. 
That air iffuing from water, or rather from the 
foft earth, or mud, at the bottom of pits con- 
taining water, is not always unwholefome, I have 
alfo had an opportunity of afcertainitig. Taking 
a walk, about two years ago, in the neighbour- 
hood of Wakefield, in Yorkshire, I obferved 
bubbles of air to arife, in remarkably great plenty, 
from a fmall pool of water, which, upon in- 
quiry, I was informed had been the place, where 
fome perfons had been boring the ground, in order 
to find coal. Thefe bubbles of air having ex- 
cited my curiofity, I prefently returned, with a 
bafon, and other veflels proper for my purpofe, 
and having ftirred the mud with a long flick, I 
foon got about a pint of this air ; and, examining it, 
found it to be good common air; at leaf! a candle 
burned in it very well. I had not then difcovered 
the method of afcertaining the goodnefs of com- 
mon air, by a mixture of nitrous air. Previous to 
the trial, I had fufpecfed that this air would have 
been found to be inflammable. 
I fhall conclude this letter with obferving, that 
I have found a remarkable difference in different: 
kinds of water, with refpedl to their effect on. 
common air agitated in them, and which 1 am 
not yet able to account for. If I agitate common 
air in the water of a deep well, near my houfe in 
Caine,, 
