2P4 ABSORPTION OP THE GASES. 



Natural waters 10. Pare diftilled water, rain and fpring water ufually con- 

 th/dlTcftareof *^"^ "^^""'y ^'^^'"^ ^"^ ^'^re of atmofpheric air: if not, they 

 atmof.airj but quickiy acquire that ihare by agitation in it, and lofe any other 

 SsTftTr^^o ^^^ ^^^^ "^^^ ^® impregnated with. It is remarkable, how- 

 oxjjen. ever, that water by ftagnation, in certain circumftances, lofes 



part or all of its oxygen, notwithftanding its conftant expo- 

 fition to the atmofphere. This I have uniformly found to be 

 the cafe in ray large wooden pneumatic trough, containing 

 about eight gallons, or 14- cubic foot of water. Whenever this 

 is replenifhed with tolerably pure rain water, it contains its 

 ihare of atmofpheric air; but in procefs of time it becomes 

 deficient of oxygen : In three months the whole furface has 

 been covered with a pellicle, and no oxygenous gas what- 

 ever was found in the water. It was grown offenfive, but 

 not extremely fo; it had not been contaminated with any ma- 

 teria! portion of metallic or fulphureous mixtures, or any other 

 article to which the effefl could be afcribed.* The quantity 

 of azotic gas is not materially diminidied by ftagnation, if at 

 all. — Thefc circumftances, not being duly noticed, have been 

 the fource of great diverfity in the refults of different philofo- 

 phers upon the quantity and quality of atmofpheric air in water. 

 By article 4-, it appears that atmofpheric air expelled from 

 water ought to have 38 percent, oxygen ; whereas by this ar- 

 ticle air may be expelled from water that (ball contain from 38 

 to per cent, of oxygen. The difappearance of oxygenous 

 gas in water, I prefume, muft be owing to fome impurities 

 in the water which combine with the oxygen. Pure rain 

 water that had flood more than a year in an earthenware bot- 

 tle had loft none of its oxygen. 

 Why water by 11. If water free from air be agitated with a fmall portioa 

 Sft i!xygtn°'''' °'' atmofpheric air (as ^\ of its bulk) the refiduum of fuch 

 from air. air will have proportionally lefs oxygen that the original : If 



we take ^, as above, then the refiduum will have only 17 

 per cent, oxygen ; agreeably to the principle eftablifhed in 

 article 4. This circumftance accounts for the obfervations 

 made by Dr. Prieftley, and Mr. William Henry, that water 

 abforbs oxygen in preference to azote. 

 Difappearance of 12. If a tall glafs vcflel, containing a fmall portion of gas 

 under^abT.'^'°" ^^ inverted into a deep trough of water, and the gas thus con- 

 fiued by the glafs and the water be brifkly agitated, it will 

 gradually difappear. 



* It was dra^un from a leaden ciftem. 



It 



