of Weiss ^ Mohs^ and Haiiy. 7S 
But these properties are easily deduced from the sign given 
by Haiiy for the same modification ; and no greater difiScul- 
ty is met with in any other case. The signs of Hauy, as 
well as those of Professors Mohs and Weiss, express some pro- 
perty of the new modification : they designate, as I mentioned 
before, in what proportion the edges of the primitive are cut by 
a plane parallel to that modification. Since some property of 
the modification must be used to designate it, why, when these 
properties are so easily deducible from each other, should the one 
be preferred to the other? It appears to me, besides, that the 
method of designation of Haiiy has the advantage of shewing, in 
the most distinct manner of the three, the position of the secon- 
dary planes, with respect to those of the primitive. One incon- 
venience peculiar to the method of Professor Mohs is, that the 
same modification may be sometimes designated by two different 
signs ; and two different forms, as to their position with the pri- 
mitive, appear to me to be represented by the same sign. The 
property of a modification having its planes tangent to the edges 
of another, and that of the simple ratio of the axis of a secondary 
form to that of the primitive, when the horizontal projections of 
both are equal, which form the base of the signs of this method, 
though simple enough in the case of the rhomboid, or of octo- 
hedrons with square or rhombic bases, seem far from being so 
in every other case. It appears, also, that the observation of a 
secondary form in a particular substance, will occasionally intro- 
duce a new series in the system of crystallisation to which that 
substance belongs. 
As to the relative simplicity of these notations, Haiiy’s me- 
thod has certainly no disadvantage. The notation of Professor 
Weiss is more uniform, but not so simple; and that of Profes- 
sor Mohs ^ does not appear simpler, if so simple. 
From the preceding considerations, I would conclude, that 
there is not apparently sufficient ground to prefer either of the 
two new methods to that of Haiiy ; that this last mode of de- 
signation may require some alterations, but that they can be in- 
* Annexed to this paper will be found the description of a crystal of Eudya- 
lite, for which I have given both the signs of Haiiy and Professor Mohs. It is 
is one of the simplest cases of Molis, all the rhomboids of that form belonging to 
the same series. 
