XXII. On ‘the Classification of Crystalline Combina- 
tions, and the Canons by which their Laws of 
Derivation may be investigated. 
By tHe Rev. W. WHEWELL, M.A. F.R.S. 
FELLOW AND TUTOR OF TRINITY COLLEGE, AND SECRETARY OF THE CAMBRIDGE 
PHILOSOPHICAL SOCIETY. 
[Read Nov. 13, 1826.] 
INTRODUCTION. 
Iv is possible so to classify crystallme forms, and so to 
consider them with respect to certam fundamental forms, that 
our reasonings, with réspect to the laws by which their planes 
are determined, shall be greatly simplified and facilitated. 
For this purpose all crystalline forms are supposed to be de- Chespisesaen 
rived from right pyramids as fundamental forms; and they are 
classified into four systems, according to the fundamental form. 
1. The Rhombohedral, in which forms are derived from 
a pyramid, having for its base an equilateral triangle. 
2. The Square-Pyramidal, in which forms are derived from 
a pyramid, having for its base a square. 
3. The Oblong-Pyramidal, in which forms are derived from 
a pyramid, having for its base a rhombus. 
4. The Octahedral, in which forms are derived from a re- 
gular octahedron. 
In all cases all the planes are supposed to be formed which 
belong to the symmetry of the figure. 
The Laws of crystalline derivation are two; the first, or Laws of De- 
law of right derivatives; and the second, or law of scalene de- Poti na 
riwatives. 
