328 Mr. Hatchett’s Analysis 
some variation ; but the composition of the former, he regards 
as fixed by the invariable law of proportions;* although he 
observes, that it has not as yet been discovered in the mineral 
kingdom .-f 
In support of these assertions, Mr. Proust states, 
1. That the pyrites found near Soria, when distilled in a 
retort heated to redness, afforded nearly 20 percent, of sulphur. 
2. That the residuum of the above distillation, had lost the 
external characters and chemical properties of pyrites, and had 
assumed those of the artificial sulphuret of iron. 
3. That when to this residuum a quantity of sulphur was 
added, and the whole was distilled in a degree of heat not too 
great, the 20 per cent, of sulphur, which had been separated by the 
first distillation, was, by this, again restored ; and the mass in the 
retort thus recovered nearly the original colour, lustre, and che- 
mical properties of the pyrites. 
4. That, by adding sulphur to iron filings, or fine iron wire, 
heated to a low red in a retort, a compound is obtained, in 
which the proportion of sulphur amounts only to about 20 or 
30 parts ; but, if this compound is again treated with sulphur 
in a red heat, a sulphuret is formed, which is readily dissolved 
in acids, and plentifully affords sulphuretted hydrogen gas. 
This is the real minimum of the sulphurets of iron, fixed by 
the invariable law of proportions, (according to Mr. Proust,) 
at 59 or 60 of sulphur and 100 of iron, the former being (as I 
have already observed) in the proportion of 37.50 per cent. 
5. and lastly. That when this sulphuret is again mixed and 
# Journal de Physique, Tome LIJI. p. 90. 
f “ La regne mineral, jusqu’ici, ne nous a point encore presente le fer sulfure aa 
minimum.” Journal de Physique, Tome LIV. p. 93. 
