I2fi The Ottawa Naturalist. 



Mohs also shewed that since all the similar edges and solid angles 

 of his fundamental figures were to be similarly altered, the existence of 

 one derived plane necessitated, as in Rome Delisle's theory, the 

 simultaneous existence of a number of others having definite positions. 

 Such a set of faces he called a simple form. If the faces of more than 



one simple form are present, the resulting form was termed a 

 combination. 



At this time Sir David Brewster was engaged in his wonderful 

 researches on the optical properties of crystals, and the results of his 

 experiments on the polarization of light brought out in such a remark- 

 able manner the intimate relations existing between their behaviour 

 with regard to light passing through them, and the number of kinds of 

 axes they possessed, that Whewcll has justly said, "Sir D. Brewster's 

 optical experiments must have led to a classification of crystals into the 

 above systems, or something nearly equivalent, even if the crystals had 

 not been so arranged by attention to their forms." 



Sometimes crystals were observed by both Weiss and Mohs which, 

 instead of being complete simple forms, like the regular octahedron, 

 presented only half the regular number of faces, as, for example, the 

 regular tetrahedron, which may be derived from the regular octahedron 

 by suppressing its alternate faces. Delisle and Haiiy hid regarded the 

 tetrahedron as a distinct kind of primitive form, but Weiss and Mohs 

 found it necessary to postulate that simple forms may not only be 

 complete, but semi-complete also, pointing out, however, that the 

 half which presents itself is not an arbitrary one, but can always be 

 derived systematically from the complete simple form 



The complete simple forms were termed holohedral y and the semi- 

 complete ones hemihedral. 



In 1822, Mohs added two more systems of crystallization to the 

 four already described by Weiss and himself; but Weiss brought forward 

 very strong objections to their recognition, and their independance was 

 not fully established until 1833, when the actions on light of crystals 

 belonging to these systems were first studied. They were what we now 

 call the monocUnic and tiiclinic systems. 



The researches of Weiss and Mohs may be said to have given to 



