106 Prof. Weiss on the Methodical and Natural Distribution 
enriched, by the particular friendship of M. Brochant, with 
Tables and an Index, to facilitate the study of them. I also 
gave an account of them to your celebrated colleague Profes- 
sor Jameson. In these memoirs you will find almost all the 
essential foundations of the actual arrangement which has now 
risen out of them in a more perfect state. You will find, for 
example, that I had established the reunion of the systems 
which have, for their primitive form, a prism with a square 
base, with those which have the octohedron with a square base, 
a union which, if I am not mistaken, you have also established 
optically. In like manner, my respectable friend M. Seebeck 
found, from his first inquiries into the optical properties of crystals, 
that they were intimately connected with the differences of the 
crystalline systems which I had proposed in 1809. In my memoir 
of 1815, I first abandoned the ideas generally entertained, of 
the Di-Rhomboedral) ; the systems of the Octohedron with a 
what is called the Primitive Form. I afterwards, in 1809, sub- 
divided them into four great divisions, separated from one an- 
other, viz. the Regular System ; the Rhomboedral Systems (with 
square base , and the systems which I then called the Octohed- 
ron with an elongated rectangle for their base, of which the 
two last are evidently the same with the systems improperly 
called by M. Mohs the Pyramidal and the Prismatic. I have 
reduced all these systems to their true source, that is, to the 
ratio of their principal axes, on which all their properties de- 
pend. I perceived, in short, since my dissertations of 1809, 
that there was in reality no exception to the four general divi- 
sions which I had then justly established. For, in 1809, it ap- 
peared to me, that feldspar , epidote , gypsum , axinite , and sul- 
phate of copper could not be referred to them ; and, for this 
reason, I then separated them from all others, in order to treat 
them apart, which I have since done in an opposite way, but 
one much more satisfactory. 
When M. Mohs came to Freyberg to succeed Werner, I do 
not suppose that he had then read my memoir of 1815. He 
had not in reality any other ideas respecting the methodical di- 
vision of the different systems of crystallization, but those which 
I had explained in 1809. I know, for example, that M. Mohs, 
in 1818, still comprehended in his prismatic system, without 
