VOL. LXXIV.] PHILOSOPHICAL TRANSACTIONS. 513 



air be mixed over dry quicksilver, the common air is not at all diminished, that 

 is, the bulk of the mixture will be not less than that of the common air em- 

 ployed, till water is admitted, and the mixture agitated for a few minutes. The 

 reason of this in all probability is, that part of the phlogisticated nitrous acid, 

 into which the nitrous air is converted, rema-ins in the state of vapour till con- 

 densed by the addition of water. A proof that this is the real case is, that, in 

 this manner of performing the experiment, the red fumes produced on mixing 

 the airs remain visible for some hours, but immediately disappear on the addition 

 of water and agitation. 



The most material experiment alleged by Mr. Kirvvan is one of Dr. Priestley's, 

 in which he obtained fixed air from a mixture of red precipitate and iron filings. 

 This at first seems really a strong argument in favour of the generation of fixed 

 air; for though plumbago, which is known to consist chiefly of that substance, 

 has lately been found to be contained in iron, yet one would not have expected it 

 to be decompounded by the red precipitate, especially when the quantity of pure 

 iron in the filings was much more than sufficient to supply the precipitate with 

 phlogiston. The following experiment however shows that it was really decom- 

 pounded; and that the fixed air obtained was not generated, but only separated 

 by means of this decomposition. 



500 gr. of red precipitate mixed with 1000 of iron filings yielded, by the assist- 

 ance of heat, 7800 gr. measures of fixed air, besides 2400 of a mixture of de- 

 phlogisticated and inflammable air, but chiefly the latter. The same quantity of 

 iron filings, taken from the same parcel, was then dissolved in diluted oil of 

 vitriol, so as to leave only the plumbago and other impurities. These mixed with 

 500 gr. of the same red precipitate, and treated as before, yielded g200 gr. mea- 

 sures of fixed air, and 4200 of dephlogisticated air, of an indifferent quality, 

 but without any sensible mixture of inflammable air. It appears therefore, that 

 less fixed air was produced when the red precipitate was mixed with the iron 

 filings in substance, than when mixed only with the plumbago and other impu- 

 rities; which shows that its production was not owing to the iron itself, which 

 seems to contain no fixed air, but to the plumbago, which contains a great deal. 

 The reason, in all probability, why less fixed air was produced in the first case 

 than the latter is, that in the former more of the plumbago escaped being de- 

 compounded by the red precipitate than in the other. It must be observed how- 

 ever, that the filings used in this experiment were mixed with about -^ of their 

 weight of brass, which was not discovered till they were dissolved in the acid, 

 and which makes the experiment less decisive than it would otherwise be. The 

 quantity of fixed air obtained is also much greater than, according to Mr. Berg- 

 man's experiment, could be yielded by the plumbago usually contained in 1000 

 gr. of iron; so that though the experiment seems to show that the fixed air was 

 vol. xv. 3 U 



