LETTER TO DR. CRELL 267 



atrial acid. 1 If these experiments were repeated with 

 copper, they would be more convincing ; but the filings of 

 copper must be free from dust and all other impurities. 



1 Mr. Cavendish lias likewise made the same observation on this 

 experiment ; and Mr. Kirwan, when he came to repeat the experiment 

 himself, for it was not originally his, as Mr. Scheele seems to suppose, 

 but Dr. Priestley's, found upon distilling recent and clean iron-filings 

 with red precipitate, or precipitate per se, in the proportion of two to one, 

 that no aerial acid nor any other air is produced, but the whole unites 

 with the iron which is calcined, while the mercury passes over. (Journal 

 de Phys., Aout, 1785, p. 146.) 



Upon this Dr. Metherie observes, that the appearance of air depends 

 on the quantity of precipitate and iron-filings employed. I have distilled, 

 says he, equal quantities of these substances in a mercurial apparatus ; 

 and a little air has appeared, which might probably have been that con- 

 tained in the vessels ; it was, however, more impure than atmospheric air. 

 The mercury was revivified, and arose into the neck of the retort. There 

 was neither any extrication nor absorption of air, and the neck of the 

 retort was covered with a considerable quantity of moisture ; the filings 

 were calcined, and lime-water introduced into the retort was precipitated. 

 On repeating the experiment, with 2 ox. of precipitate and 72 grs. of 

 filings, a quantity of air was extricated, which contained a little aerial 

 acid, but consisted chiefly of pure air. T. 



