128 M. Beudant on Mineral Species, [Fes. 
three, or six and six, in relation to the same line, which passes 
through two solid opposite angles. This appears to me ver 
constant, both in all the crystals which I have myself obtained, 
or in those which our manufactories of this salt daily produce. 
The figures which M. Haiuy has given of this salt (plate 79) are 
perfectly accurate, and it is sufficient to cast the eye on them to 
see that the faces marked x, 0, s, 7, x, are all arranged relative. 
to an axis which joins the two solid acute angles. This axis, 
therefore, indicates a pyramidal system of crystallization (if the 
expression be allowable), and which, therefore, is quite foreign 
to a system of an oblique prism. When this last system of 
crystallization exists, the symmetry of the modifications is quite 
different. The faces which modify the prevailing form are never 
arranged relatively to an axis passing through two opposite solid 
angles, and of course the edges, or angles, on which these modi- 
fications take place, have not that symmetry which a rhomboid 
presents. 
The expression linear dimensions, which Dr. Wollaston 
employs, seems to indicate that he has taken into consideration 
the relative dimensions which the edges of the crystals obtained, 
present. This I recognize in a subsequent part of the letter, 
when he mentions an experiment quite similar to one of those 
which I gave in the last memoir that I presented to the Academy 
of Sciences (Recherches sur les Causes qui font varier les 
Formes Crystallines d’une meme Substance Minerale), in which 
he obtained, as I did, very elongated sulphate of iron mixed 
with copper. He says on this subject, “ the prismatic form is so 
elongated that it shows evidently that it is not a rhomboid.” 
I cannot by any means agree to this opinion. A rhomboid_ 
_may be elongated in one direction; it then exhibits a kind of 
oblique prism with a rhomboidal base. But though the edges 
are unequal in their dimensions, the rhomboidal character does 
not exist the less; but is recognized in the symmetry of ‘the 
modifications. In fact, I obtain at pleasure these same crystals, 
elongated, and modified by additional faces; and I recognize 
obviously that all these faces are placed symmetrically with 
respect to one axis—a character which accurately characterizes 
the rhomboidal system. 
2. Dr. Wollastom seems to think that I consider the crystals 
which I obtained as mechanical mixtures of different other salts, 
in a manner analogous to the gres de Fontainebleau. I never 
entertained any such idea; and one of the notes attached to my 
memoir shows this sufficiently. I think likewise that we may 
regard these associations as combinations ; but since they take 
pace in variable proportions, it was necessary to distinguish them 
rom combinations in definite proportions. This was the reason 
why I adopted the expression chemical mixture. 
3. Dr. Wollaston mentions experiments in which he dissolved 
