VOL. LXXIV.] PHILOSOPHICAL TRANSACTIONS. 505 



paper : whichever way this is explained, some or other of my opinions are con- 

 firmed ; for either the mercurial calx is already combined with fixed air, which I 

 believe to be the case, and this air passes undecomposed, because the mercury 

 extracts phlogiston from the iron ; or it contains dephlogisticated air, which is 

 converted into fixed air by its union with the phlogiston of the iron. 



If precipitate per se be digested in marine acid, the mercury will be revived 

 (3 Bergm. 415.) Now this calx does not dephlogisticate the marine acid; for 

 this acid, when dephlogisticated, dissolves mercury ; how then does it revive it, 

 if not by expelling the fixed air contained in it, which in the moment of its ex- 

 pulsion is decomposed, leaving its phlogiston to the mercury, which is thus re- 

 vived? Again: if litharge be heated in a gun-barrel, it will afford more fixed 

 and less dephlogisticated air, than if heated in glass or earthen vessels. Does 

 not this happen, because the calx of lead, receiving some phlogiston from the 

 metal, does not dephlogisticate so great a proportion of the fixed air as it other- 

 wise would ? 



Further : there is no substance which yields dephlogisticated air, but yields 

 also fixed air, even precipitate per se not excepted (3 Priest. 1 6;) and, what is 

 remarkable, they all yield fixed air first, and dephlogisticated air only towards 

 the end of the process. Does not this happen because metallic calces attract 

 phlogiston so much more strongly, as they are more heated ? Thus many calci- 

 form iron ores become magnetic by calcination, though they were not so before ; 

 so also do all the calces of iron when exposed to the focus of a burning glass 

 (5 Diet. Chy. 179.) Thus mercury cannot be calcined but in a heat inferior to 

 that in which it boils ; thus minium cannot be formed but in a moderate heat ; 

 and if heated still more, it returns to the state of massicot, in which it was be- 

 fore it became minium, and much of it is reduced. So if a solution of luna 

 cornea in volatile alkali be triturated with mercury, the silver will be revived, 

 and the marine acid unite to the mercury ; which shows this acid has a stronger 

 attraction to mercury than to silver; yet if sublimate corrosive and silver be dis- 

 tilled in a strong heat, the mercury will be revived, and the marine acid unite to 

 the silver ; which shows that the attraction of mercury to phlogiston increases 

 with the heat applied. 



Before concluding this head, I will mention another experiment, which I 

 think decisive in favour of my opinion of the composition of fixed air. If filings 

 of zinc be digested in a caustic fixed alkali in a gentle heat, the zinc will be dis- 

 solved with effervescence, and the alkali will be rendered in great measure mild. 

 But if, instead of filings of zinc, flowers of zinc be used, and treated in the 

 same manner, there will be no solution, and the alkali will remain caustic. In 

 the first case the effervescence arises from the production of inflammable air, 

 which phlogisticates the common air contiguous to it, and produces fixed air, 



vol. xv. 3 T 



