Analysis of the magnetical Pyrites. 1S9 



required to form the sulphuric acid contained in 100 pans 

 of sulphate of barytes at 14*5 *, it follows, that 24 j grains 

 of dry sulphate of barytes contain sulphuric acid equal, verv 

 nearly, to 36 grains of sulphur ; so that the two analyses 

 corroborate each other. Tlie proportion of sulphur in the 

 jiiagnetical pvrites may therefore be stated at 3(3-50, or in- 

 deed at 37 per cent, if some small aliowaace be made for 

 the occasional presence of earthy paiticles; a Uiinutt por- 

 tion of qi^artz having been found, by tlic last analysis, after 

 the complete acidiricatioi: of the sulphur. 



The increase produced, by the operalions of the analvsis, 

 in the weif^ht of the iron, arose, as I have alreadv remarked, 

 from the addition cf oxygen ; ff-r the iron, as obtained by 

 the analysis, was in the state of black oxide ; but in this, 

 and indeed in all pvrites, it undoubtedly exists very nearly, 

 or quite, in the st ae of pericct metal. Now the black oxide 

 of iron, called firutoxide by Dr. Thomson f, has been proved 

 bv Lavoisier and I'roust to consist of 100 parts of metallic 

 iron combined with 37 of oxygen, thus forming 137 of 

 black oxide : the exact proportion of oxygen is therefore 

 -21 per cent., and SO grains of this oxide must contain 21*6 

 of oxygen. But, in the above analyses of the niaoneiical 

 pyrites, the increase of weight did not amount to more 

 than \Q-b; and we may therefore conclude that, in all 

 probability, a quantity of oxygen = 5*1 was previously 

 combined with some part, or with the general mass, of the 

 iron in the pyrites. A small part of the above-mentioned 

 increase of weight must likewise have arisen from another 

 cause ; for, allliough the true proportions of the black oxide 

 of iron are 27 of oxygen and 73 of iron, (so that 100 parts 

 uf the latter absorb 37 of the former,) yet, in actual prac- 

 tice, it is difficult to obtain it exactly in this state, and there 

 is comnionl}- a small excess of weight : this I have repeat- 

 edly observed in many experiments, some of which were 

 purposely made. When, for instance, 100 parts of fine iron 

 wire were dissolved in muriatic acid, and afterwards preci- 

 pitated by annnonia, edidcorated, dried, and uKidc red-hot 

 with a small (piantity of wax in a covered porcelain cruci- 

 ble, the weiglit, in'^tead of 137, usually amounted to 139 

 or 140. The quantity of wax empU)yed certainly did not 

 alTbrd a ponderable quantity of coal or other residuum ; but 

 the real cause of the increase of weight appears tu be the 

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



* Transactions of the Royal Irish AcnJ«-my, vol. viii. p. 14.0. 

 + S)$cc[n of Cbcmiitry, id cdiiion, vol. i. p. 147. 



after 



