C50 



V. The Bakerian Lecture. 0?i the elementary Particles of certain 

 Crystals. By William Hyde Wollaston, M. D. Sec. R.S. 



Read November 26, 1812. 



Among the known forms of crystallized bodies, there is no 

 one common to a greater number of substances than the 

 regular octohedron, and no one in which a corresponding 

 difficulty has occurred with regard to determining which mo- 

 dification of its form is to be considered as primitive ; since in 

 all these substances the tetrahedron appears to have equal 

 claim to be received a* the original from which all their other 

 modifications are to be derived. 



The relations of these solids to each other is most distinctly 

 exhibited to those who are not much conversant with crystal- 

 lography, by assuming the tetrahedron as primitive, for this 

 may immediately be converted into an octohedron by the 

 removal of four smaller tetrahedrons from its solid angles. 



(Fig. 1.) 



The substance which most readily admits of division by 

 fracture into these forms is fluor spar ; and there is no diffi- 

 culty m obtaining a sufficient quantity for such experiments. 

 But it is not, in fact, either the tetrahedron or the octohedron, 

 which first presents itself as the apparent primitive form ob- 

 tained by fracture. 



If we form a plate of uniform thickness by two successive 

 divisions of the spar, parallel to each other, we shall find the 



H2 



