VOL. LXXIV.] PHILOSOPHICAL TRANSACTIONS. 50y 



remark, that Dr. Priestley did not indeed at first prepare this powder on purposes 

 but he afterwards did so prepare it (4 Priest, p. 148, 149) and obtained a powder 

 exactly of the same sort; and it is certain that the fixed air found in it proceeded 

 from the common air, both because metallic calces, not formed by amalgamation, 

 will not unite with mercury, as is well known; and because this calx cannot be 

 formed by agitation of the mercury and lead, in phlogisticated, inflammable, or 

 any other air which is not respirable; and the fixed air cannot proceed from any 

 impurity, as mercury will not unite in its running form to any other but metallic 

 substances, which it always partially dephlogisticates, like other menstruums 

 (3 Chy. Dijon, 425). 



Of the diminution of respirable air by combustion. — Though I have no doubt 

 but the diminution of respirable air, by the combustion of sulphur and phos- 

 phorus, proceeds also in great measure from the production and absorption of 

 fixed air, yet I avoided mentioning this operation, as the presence of a stronger 

 acid renders the presence of a weaker impossible to be proved, more especially as 

 both these acids precipitate lime from lime-water; but the great increase of 

 weight which the phosphoric acid gains is a strong additional inducement to 

 think that it absorbs fixed air. During the combustion of vegetable substances, 

 I think it highly probable that fixed air is formed, both from my own experi- 

 ments on the combustion of wax candles, and that mentioned in the 1st volume 

 of Dr. Priestley's Observations, p. 13d; but when inflammable air from metals 

 and dephlogisticated air are fired, as a great diminution takes place, and yet no 

 fixed air is found, I am nearly convinced, by Mr. Cavendish's experiments, that 

 water is really produced; nor am I surprized that, in this instance, the union of 

 phlogiston and dephlogisticated air should form a compound very different from 

 that which it forms in other instances of phlogistication, but should rather be led 

 to expect it a priori; for in this case the phlogiston is in its most rarefied known 

 state, and unites to dephlogisticated air, the substance to which it has the 

 greatest affinity, in circumstances the most favourable to the closest and most 

 intimate union ; for both, in the act of inflammation, are rarefied to the highest 

 degree; both give out their specific fire, the great obstacle to their union, it 

 being by the inflammation converted into sensible heat (a circumstance which, in 

 my opinion, constitutes the very essence of flame); the resulting compound 

 having then lost the greatest part of its specific fire, is necessarily reduced, ac- 

 cording to Dr. Black's theory, into a denser state, which the present experiment 

 shows to be water; whereas, in common cases of combustion, the phlogiston 

 being denser and less divided, unites less intimately with the dephlogisticated part 

 of common air, consequently expels less of its specific fire, and therefore forms 

 less dense compounds, viz. fixed and phlogisticated airs; and so much the more, 

 as a great part entirely escapes combustion; but it seems probable that in 



