5 6 Count de Bournon’s Description of a 
Among the above forms, there is one in particular, which has 
not yet been described as belonging to this sulphuret; namely, 
the dodecaedron with rhombic planes, and also the passage of 
the tetraedron, more or less advanced, towards this dodecaedron, 
by each of its solid angles having been replaced by three planes, 
situated upon its sides. This dodecaedron, which is by no means 
common, even in Cornwall, (the only place where I have yet 
met with it,) is found there of a pretty considerable size, being 
sometimes an inch, or even more, in diameter. This form has 
never yet been observed among the sulphurets of iron. 
All the other characters of the tetraedral yellow double 
sulphuret, are likewise different from those of the octaedral sul- 
phuret of iron. Its yellow colour is more deep ; its fracture is 
more brilliant ; its grain is much less even, and exhibits some 
chance. In like manner, as those sulphurets of iron which contain neither copper nor 
gold, are found in the form of octaedrons, of striated cubes, and of dodecaedrons, it is 
evident that the form of such sulphurets is not modified by either of tire last-mentioned 
metals. 
The above observations, in my opinion, tend very much to confirm what I have 
advanced with respect to the heterogeneous attraction of aggregation, which appears (in 
an infinite number of circumstances) to take place between the integrant molecules of 
one substance and others that are of a nature totally different, and gives rise to a body 
that is- physically, but not chemically, different from what would have been produced, 
rf these heterogeneous molecules had not been interposed within it. 
Thus, even the sulphuret of iron generally contains a much larger proportion of 
sulphur than that which combines with the iron, during its formation. This superfluous 
proportion of sulphur is consequently foreign to the nature of the sulphuret, and is 
not necessary to its formation. It may be separated by means of distillation, without 
decomposing the real sulphuret ; and manifests itself in a very striking manner, when, 
after having grossly powdered the sulphuret, a portion of the powder is thrown upon 
a live coal, or a red hot iron, by the inflammation which takes place, on account of this 
portion of the sulphur being in an uncombined state. All kinds of pyrites, when treated 
in this manner, emit a phosphorescent light, of a fiery red colour. 
