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the experiment of the Black Hole were to be re- 

 peated, a man would ftand the better chance of fur- 

 viving it, who mould 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- 

 stances in which an old moufe has not lived one r 

 On thefe accounts, experiments with mice, and, for 

 the fame reafcn, no doubt, with other animals alfo, 

 have a confiderable degree of uncertainty attending 

 them ; and therefore, it is necefTary to repeat them 

 frequently, before the refult can be abfohitely depend- 

 ed upon. 



The difcovery of the provifion in nature for re- 

 floring air, which has been injured by the refpiration 

 of animals, having long appeared to me to be one of 

 the mofl important problems in natural philofophy, 

 I have tried a great variety of fchemes in order to 

 efFecl: it. In thefe, my guide has generally been to 

 eonfider the influences to which the atmofphere is, 

 in fact, 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- fubjecl, I 

 /hall mention the principal of them-. 



The noxious effluvium with which air is loaded 

 by animal refpiration, is not abfbrbed by {landing 

 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 reflore it, by 

 the methods which will be explained hereafter, than 



