[ i8 3 ] 
the experiment of the Black Hole were to be re- 
peated, a man would Hand the better chance of fur- 
viving it, who fhould enter at the firft, than at the 
laft hour. I have alfo obferved, that young mice 
will always live much longer than old ones, or than 
thofe which are full grown, when they are confined 
in the fame quantity of air. I have fometimes known 
a young moufe to live fix hours in the fame circum- 
flances in which an old moufe has not lived one. 
On thefe accounts, experiments with mice, and, for 
the fame reafon, no doubt, with other animals alfo, 
have a confiderable degree of uncertainty attending 
them j and therefore, it is necelfary to repeat them 
frequently, before the refult can be abfolutely depend- 
ed upon. 
The difcovery of the provifion in nature for re- 
ftoring air, which has been injured by the refpiration 
of animals, having long appeared to me to be one of 
the moil important problems in natural philofophy, 
I have tried a great variety of fchemes in order to 
effedt it. In thefe, my guide has generally been to 
confider the influences to which the atmofphere is, 
in fadt, expofed ; and, as fome of my unfuccefsful 
trials may be of ufe to thofe who are difpofed to take 
pains in the farther inveftigation of this fubjedl, I 
fhall mention the principal of them. 
The noxious effluvium with which air is loaded 
by animal refpiration, is not abforbed by Handing 
without agitation in frefh or fait water. I have kept 
it many months in frefh water, when, inftead of 
being meliorated, it has feemed to become even more 
deadly, fo as to require more time to reftore it, by 
the methods which will be explained hereafter, than 
