3 2 S 
of the magnetical Pyrites, &c. 
The increase produced, by the operations of the analysis, in the 
weight of the iron, arose, as I have already remarked, from the 
addition of oxygen ; for the iron, as obtained by the analysis, 
was in the state of black oxide ; but in this, and indeed in all 
pyrites, it undoubtedly exists very nearly, or quite, in the state of 
perfect metal. Now the black oxide of iron, called Protoxide by 
Dr. Thomson,* has been proved, by Lavoisier and Proust, to 
consist of 100 parts of metallic iron combined with 3 7 of oxygen, 
thus forming 137 of black oxide ; the exact proportion of oxygen 
is therefore 27 per cent, and 80 grains of this oxide must contain 
21.6 of oxygen. But, in the above analyses of the magnetical 
pyrites, the increase 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 previously combined with some 
part, or with the general mass, of the iron in the pyrites. A small 
part of the abovementioned increase of weight, must likewise 
have arisen from another cause ; for, although the true propor- 
tions of the black oxide of iron are 27 of oxygen and 73 of iron, 
(so that 100 parts of the latter absorb 37 of the former,) yet, in 
actual practice, it is difficult to obtain it exactly in this state, 
and there is commonly a small excess of weight : this I have 
repeatedly 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 precipitated 
by ammonia, edulcorated, dried, and made red-hot with a small 
quantity of wax in a covered porcelain crucible, the weight, 
instead of 137, usually amounted to 139 or 140. The quantity 
of wax employed, certainly did not afford a ponderable quantity 
of coal, or other residuum ; but the real cause of the increase of 
* System of Chemistry, 2d edition, Vol. I. p. 147, 
