504 PHILOSOPHICAL TRANSACTIONS. [ANNO 1784. 



affinity, for if a saturate solution of mercury in any of the acids be precipitated 

 by a mild vegetable alkali, very little effervescence is perceived, and the precipi- 

 tate weighs much more than the quantity of mercury employed, and that this 

 increase of weight arises in part from the fixed air absorbed will presently be 

 seen. 



Since then metals may be calcined in close vessels ; since they then absorb a 

 4th part of the common air to which they are exposed ; since all metallic calces 

 (except those of mercury, which I shall presently mention) yield fixed air ; since 

 common air contains scarce any fixed air ; is it not apparent that the fixed air 

 thus found was generated by the very act of calcination, by the union of the 

 phlogiston of the metal with the dephlogisticated part of the common air, since 

 after the operation the metal is deprived of its phlogiston, and the air of its de- 

 phlogisticated part ? But Mr. Cavendish objects, that no one has extracted fixed 

 air from metals calcined in close vessels. To which I answer, that this further 

 proof is difficult, and no way necessary ; it is difficult, because the operation 

 can easily be performed only on small quantities ; it is unnecessary, because it 

 differs from the operation in open air only by the quantities of the materials 

 employed ; in every other respect it is exactly the same. Since Mr. Cavendish 

 suspects the results are different, it is incumbent on him to show that difference; 

 but till then, according to Sir Isaac Newton's second rule, " to natural effects 

 of the same kind the same causes are to be assigned, as far as it may be done," 

 that is, till experience points out some other cause. 



It may further be urged, that precipitate per se yields only dephlogisticated 

 air, that minium also yields a large proportion of it. This difficulty I have for- 

 merly answered by asserting, that these calces are in fact united only to fixed air, 

 and that they yield dephlogisticated air, merely because the fixed air is decom- 

 posed by the total or partial revivification of the metallic substances; this I think 

 may be demonstrated by the following experiments. Let sublimate corrosive 

 singly be treated in any manner, it will not yield dephlogisticated air (4 Pr. 240;) 

 but let a solution of sublimate corrosive be precipitated by a mild fixed alkali, 

 this precipitate washed, dried, and distilled in, a pneumatic apparatus, will yield 

 dephlogisticated air, and the mercury will be revived ; but, if the solution of sub- 

 limate corrosive be precipitated by lime-water, it seems no air will be produced. 

 Here then we see, 1st, that the calx of mercury unites with fixed air; and, 

 2dly, that this fixed air is, during the revivification of the mercury, converted 

 into dephlogisticated air. Again : let I oz. of red precipitate, which, according 

 to Mr. Cavendish, contains no nitrous acid, be distilled with 2 oz. of iron filings; 

 this quantity of precipitate, which, if distilled by itself, would yield 00 oz. mea- 

 sures of dephlogisticated air, will, when distilled with this proportion of iron 

 filings, yield 40 oz. measures of fixed air, as Dr. Priestley has shown in his last 



