316 M. Huygcns on the cause of the 



linus informs us that he once found prisms having the form 

 of a triangular pyramid. But when a mass is only composed 

 internally of these small spheroids thus arranged, whatever 

 form it has without, it is certain, from the same reasons which 

 I have just explained, that it would cleave into similar prisms. 

 It remains to be seen if there are other reasons which confirm 

 this conjecture, and if there are any which oppose it. 



It may be objected, that if this crystal is so composed, it 

 will still cleave in other two ways, one of which will be in the 

 direction of planes parallel to ABC, the base of the pyramid, 

 and the other parallel to a plane whose section is marked by 

 the lines GH, HK, KL. To this I reply, that both these 

 cleavages, though practicable, are more difficult than those 

 which are parallel to any of the three planes of the pyramid ; 

 and, therefore, in striking the crystal in order to break it, it 

 should always cleave in preference in those three planes than 

 in the two others. If we take a number of spheroids, of the 

 form above-mentioned, and range them in a pyramid, we shall 

 see why the two divisions (parallel to ABC and GHKL) are 

 more difficult. With respect to that parallel to ABC, each 

 spheroid must detach itself from three others which touch by 

 their flat surfaces, which hold firmer than those which touch 

 by their edges ; and, besides this, this division will not take 

 place by entire strata, because every one of the spheroids of a 

 stratum is almost never retained by the six of the same stra- 

 tum which surround it, because they touch only by their 

 edges, so that it adheres easily to the neighbouring stratum, 

 and others to it for the same reason, which produces unequal 

 surfaces. We also find from experiment, that in grinding the 

 crystal on a stone a little rough, directly upon the solid equi- 

 lateral angle, great facility is experienced in wearing it down 

 in this direction, but great difficulty in polishing it. 



With regard to the other cleavage along GHKL, each 

 spheroid must be detached from Jive of the neighbouring stra- 

 tum, two of which touch by their flat surfaces, and two others 

 by their edges ; so that this cleavage is even more difficult than 

 that which is made parallel to one of the faces of the crystal, 

 where we have said that each spheroid is detached only from 



