VOL. LXXIV.] PHILOSOPHICAL TRANSACTIONS. 515 



not have happened; for, 1st, the zinc, instead of being further acted on by the 

 acid, was precipitated according to Mr. Lassone's own account, p. 8; and, 2dly, 

 the acid was only added by degrees, and undoubtedly would unite to the alkali 

 preferably to the zinc; therefore it was from the alkali, and not from the zinc, 

 that the effervescence arose. 



2dly. With regard to the calcination of lead; though in England the smoke 

 and flame may come in contact with the metal, yet in Germany red lead is formed 

 without any communication between them, according to Mr. Nose, who has 

 given an ample account of this manufactory, p. 86. Is not lime formed in con- 

 tact with fuel, flame, and smoke? Mr. Macquer even thinks it probable, that 

 the contact of flame is hurtful to the production of minium, 2 Diet. Chy. 63Q. 

 Mr. Monnet made minium by melting lead in a cuppel, in such a manner that 

 it was impossible it could come in contact with the least particle of flame or 

 smoke, Mem. Turin. 1769, p. 71. 



Mr. Cavendish expresses his surprize at my asserting, that the black powder, 

 which Dr. Priestley formed out of an amalgam of mercury and lead, was exactly 

 the same as that out of which he had extracted fixed air; but I think I have as- 

 signed very sufficient reasons for my opinion: how far I was right will best appear 

 by Dr. Priestley's own letter, in the hands of the secretary, of which the fol- 

 lowing is an extract. " I certainly imagined the two black powders you write 

 about to be of the same nature, and therefore did not attempt to extract any air 

 from the latter; but immediately on the receipt of your favour of yesterday, I 

 dissolved an ounce of lead in mercury, and expelling it by agitation, put the 

 black powder, which weighed near 1 2 oz., into a coated glass retort ; then ap- 

 plying heat, I got from it about 20 oz. measures of very pure fixed air, not T ' T 

 of which remained unabsorbed by water." 



4thly. It is impossible to attribute the fixed air, produced by the distillation of 

 red precipitate and filings of iron, to the decomposition of the plumbago con- 

 tained in the iron; for the quantity of fixed air produced in Mr. Cavendish's own 

 experiment is more than twice the weight of the whole quantity of plumbago 

 contained in the quantity of iron he used, supposing the whole of the plumbago 

 to consist of fixed air, which is not pretended; and more than 8 times the weight 

 of the quantity of fixed air, which plumbago really contains. For Mr. Caven- 

 dish employed in his experiment 1000 gr. of iron and 500 gr. of red precipitate, 

 and obtained 7800 gr. measures of fixed air, which are equal to 30 cubic inches, 

 and weigh 17 gr. Now 100 gr. of bar iron contain, according to Mr. Bergman, 

 at most, T v of a grain of plumbago: and consequently 1000 gr. of this iron 

 contain but 2 gr. of plumbago; and plumbago, according to Mr. Scheele, con- 

 tains but 4- of its weight of fixed air; so that here, supposing the plumbago to 

 be decomposed, we can have at most but -jV of a grain of fixed air, or little more 



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