324 Mr . Hatchett's Analysis 
weight, appears to be the air, which can scarcely be completely 
excluded, and which, after the wax is burned, combines with 
the superficial part of the oxide, and converts a portion of it 
into the red or peroxide; so that the surface in the crucible 
appears brown, when compared with the interior. 
To this cause, therefore, I am inclined also to attribute a 
small part of the increase observed in the weight of the iron 
obtained by the preceding analyses. 
§V. 
Before I make any observations on the nature of the sulphuret 
which has been proved to constitute the magnetical pyrites, it 
may be proper to state some comparative analyses which I have 
made, of several of the common pyrites ; and, as the method 
employed was precisely the same as that which has been de- 
scribed, all that seems to be requisite, is to give an account of 
the results. 
In each analysis, the whole of the sulphur was converted into 
sulphuric acid, which was precipitated by barytes; and, in the 
selection of the specimens, great attention was paid, to take the 
internal parts of the fragments, and not to make use of any 
which exhibited an appearance of decomposition, or of extra- 
neous substances. 
The iron was, as before, reduced to the state of black Oxide; 
and the addition of weight in each separate analysis, corresponded, 
within a few fractional parts, with the proportion of oxygen 
requisite to form into black oxide a given quantity of metallic 
iron, equal to that which in each pyrites was ascertained to be 
the real proportion, by deducting the quantity of sulphur from 
the total quantity of each pyrites. 
