XXIII. Reasons for the Selection of a Notation to 
designate the Planes of Crystals. 
By THE Rev. W. WHEWELL, M.A. F.R.S. 
FELLOW AND TUTOR OF TRINITY COLLEGE, AND SECRETARY OF THE CAMBRIDGE 
PHILOSOPHICAL SOCIETY, 
[Read Feb. 11, 1826.] 
In a communication read to the Society in the course of last 
year, I pointed out that crystals may be divided into four classes, 
according to their degree of symmetry. Being referred to an axis, 
they may consist of three similar and symmetrical portions about 
this axis, or of four, or of two opposite pairs of similar portions. 
They may also be symmetrical, with respect to three axes at right 
angles to each other. The merit of this classification has been 
a subject of controversy between Professor Mohs, and Professor 
Weiss, both of them persons to whom the science of Mineralogy 
is deeply indebted ; but to whomsoever it is to be ascribed, it is 
a division so consonant to the nature of the case, and the mode 
in which we have to reason on crystalline forms, that there can 
be no doubt of its being, for the future, the proper and scientific 
view of the subject. Founded upon this view, it may be mentioned 
also, that Professor Mohs had given modes of deducing all derived 
crystalline forms, and of designating them by a notation, such 
as to render easy and general the process of deducing the Jaws 
according to which each is formed. Whether his mode of de- 
duction is simpler than any previously proposed, is a question 
which must be determined by a consideration of the forms which 
really occur in nature, and the discussion of it must be reserved 
