339 
of the magnetical Pyrites , &c. 
not appear, from the two memoirs published by Mr. Proust, to 
which I have so frequently alluded, that that gentleman did 
more, in respect to analysis, than distil the cubic and dode- 
caedral pyrites found near Soria, from which he obtained about 
20 per cent, of sulphur; and, having observed that the residuum 
possessed the properties of the sulphuret which has been com- 
monly prepared in laboratories, he concluded that the sulphur 
obtained from the pyrites, is the excess of that proportion which 
is requisite to form the sulphuret, the proportions of which, 
therefore, he by synthesis ascertained to be, as I have above 
stated, = 37.50 of sulphur, and 62.50 of iron, or 60 of sulphur 
combined with 100 of iron; and lastly, having formed 318 
grains of this sulphuret from 200 grains of iron filings, he 
distilled the sulphuret with an additional quantity of sulphur, in 
an inferior degree of heat, and obtained 378 grains of a sub- 
stance which, excepting density, was similar to the common 
martial pyrites.* 
It is however to be regretted, that Mr. Proust did not make 
a regular analysis of the pyrites of Soria, and of the residuum 
after distillation ; for (unless these pyrites are very different from 
those which I have examined) he would most probably have 
found the proportion of sulphur greater than that which he has 
assigned to natural pyrites in general. This at least there is great 
reason to suppose, if we allow that most or all of the pyrites 
have been formed in the humid way, by which, we may 
conceive, a larger proportion of sulphur may be introduced 
into the compound, than can take place in high degrees of 
temperature. And this opinion is corroborated by the results of 
• Journal de Physique, Tome LIV. p. 92. 
