VOL. LXXIV.] PHILOSOPHICAL TRANSACTIONS. 503 



made sensible ; for if a cubic inch of it be dissolved in 3 oz. of water, a few 

 drops of that water let into lime-water will produce a cloud. Mr. Fontana says, 

 he frequently agitated 1 cubic inch of the tincture of turnsole in 7 or 800 of 

 common air, without reddening it (23 Roz. p. 188 ;) and yet, according to Mr. 

 Bergman, 1 cubic inch of fixed air is sufficient to redden 50 of tincture of turn- 

 sole (1 Bergm. 1 1^) whence I am apt to think, that 700 cubic inches of common 

 air do not even contain ^ of a cubic inch of fixed air. Dr. Whytt found that 

 12 oz. of strong lime-water, being exposed to the open air for 19 days, still re- 

 tained about 1 grain of lime, (on Lime-water, p. 32.) Now 12 oz. of strong 

 lime-water contain at most 9-5 grs. of lime ; and 1 grain of lime requires only 

 O.56 of a cubic inch of fixed air to precipitate it, the thermometer at 55 and the 

 barometer at 29.*5, as I have found. Therefore in 19 days this lime-water did 

 not come in contact with more than 4 cubic inches of tixed air ; yet it is certain 

 that a large quantity of fixed air is continually disengaged, and thrown into the 

 atmosphere, by various processes, as putrefaction, combustion, &c. but it seems 

 equally certain that it is either decomposed, or more probably absorbed by various 

 bodies. Mr. Fontana let loose 20000 cubic inches of fixed air, in a room whose 

 windows and doors were closed, yet in half an hour after he could not discover 

 the least trace of it (ibid.) Though fixed air perpetually oozes from the floor of 

 the Grotto del Cane, yet at the distance of 4 or 5 feet from the ground none is 

 found; animals may live, lights burn, &c. (Roz. Ibid. Mem. Stockh. 1775). 

 If distilled water be exposed to the atmosphere, it is never found to absorb fixed 

 air, but rather dephlogisticated air, according to Mr. Scheele's experiments, 

 which could never happen if the atmosphere contained any sensible proportion 

 of fixed air ; nor has rain-water been ever found to contain any, which it cer- 

 tainly should on the same hypothesis ; even Mr. Cavendish himself could 

 find no fixed air "in the residuum or products of about 1040 oz. measures of 

 common air, which he burnt with inflammable air. It is true, Dr. Priestley 

 supposed common air to contain -^ of its bulk of fixed air ; but he drew this 

 conclusion not from any direct experiment, but from the quantity of fixed air 

 produced by breathing, which he at that time believed to have been barely pre- 

 cipitated, and not generated, an opinion which he has found reason to alter from 

 his own experiments. I think I may therefore conclude, that the quantity of 

 fixed air contained in the atmosphere is absolutely inappreciable. 



Secondly, supposing the atmosphere to contain a very small proportion of 

 fixed air, yet I do not think it can be inferred that metals, during their calcina- 

 tion, extract any, because I find that lime exposed to red heat ever so long, ex- 

 tracts none, though it is formed by a calcination in open air, which lasts at least 

 as long as that of any metal ; neither does precipitate per se attract any, though 

 its calcination lasts several months ; nor does this proceed from the want of 



