MAGNETICAL PTRITES. 271 



indeed in all pyrites, it undoubtedly exifts very nearly,, or 

 quite, in the ftate of perfect metal. Now the black oxide of 

 iron, called Protoxide by Dr. Thornfon *, has been proved, 

 by Lavoifier and Prouft, to confiit of 100 parts of metallic 

 iron combined with 37 of oxygen, thus forming 137 of black 

 oxide ; the exact proportion of oxygen is therefore 27 per 

 cait, and 80 grains of this oxide mud contain 2J.6 of oxygen. 

 But, in the above analyfes of the magnetical pyrites, the 

 increafe of weight did not amount to more than 16.5 ; and we 

 may therefore conclude that, in all probability, a quantity of 

 oxygen = 5.1 was previoufly combined with fome part, or 

 with the general mafs, of the iron in the pyrites. A fmall 

 part of the abovementioned increafe of weight, mu(l likewife 

 have arifen from another caufe ; for, although the true propor- 

 tions of the black oxide of iron are 27 of oxygen and 73 of 

 iron, (fo that 100 parts of the latter abforb 37 of the former,) 

 yet, in actual practice, it is difficult to obtain it exactly in this 

 fiate, and there is commonly a fmall excefs of weight: this 

 I have repeatedly obferved, in many experiments, fome of 

 which were purpofely made. When, for inftance, 100 parts 

 of fine iron wire were diflblved in muriatic acid, and after- 

 wards precipitated by ammonia, edulcorated, dried, and made 

 red-hot with a fmall quantity of wax in a covered porcelain 

 crucible, the weight, inftead of 137, ufually amounted to 139 

 or 140. The quantity of wax employed, certainly did not 

 afford a ponderable quantity of coal, or other refiduum ; but 

 the real caufe of the increafe of weight, appears to be the 

 air, which can fcarcely be completely excluded, and which, 

 after the wax is burned, combines with the fuperficial part of 

 the oxide, and converts a portion of it into the red or per- 

 oxide ; fo that the furface in the crucible appears brown, when 

 compared with the interior. 



To this caufe, therefore, I am inclined alfo to attribute a 

 fmall part of the increafe obferved in the weight of the irOn 

 obtained by the preceding analyfes. 



§V. 



Before I make any obfervations on the nature of the ful- Comparative 

 phuret which has been proved to conftitute the magnetical ana 'y fis of other 



pyrites, by aci- 

 * Syftem of Chemiftry, 2d edition, Vgl. I. p. 147. >Se 8 fA e 



pyrites, ftl P hur ' 



