Mr. William Phillips on the Oxyd of Tin. Z5*l 



with both its diagonals. I presume also that it has been shewn, 

 by the agreement in the incidences of the plane P on the edge b c 

 of the fracture, fig. 9. with the plane P on the edge h c oi the 

 crystal, fig. 11. as well as of the plane P with the plane a of each 

 of those figures ; that the plane P of the former figure, is really the 

 result of a cleavage parallel with the plane P of the latter figure ; 

 and also, by fig. 10. that a mechanical division is equally practicable 

 parallel with each of the four planes P, composing the pyramid of 

 the crystal fig. 1 1 . Let therefore all these cleavages be represented 

 on fig. 15. and it will be seen that the result is a mechanical 

 division of it into tetrahedrons. 



It has already been said that in the first instance the Abbe Haiiy, 

 was induced to believe the cube to be the primitive form of the 

 oxyd of tin, but that he was afterwards led to adopt the flattened 

 octohedron composed of the two pyramids of fig. 15. joined base 

 to base. In this latter opinion, there seems to me, from the 

 evidence now offered, no room for doubting his correctness. For 

 whatever has been said tending to shew a connexion between the- 

 fractures that have been described, and a crystal delineated by 

 fig. 15. relates, with equal aptitude, to one having either a longer 

 or a shorter prism, and equally well to one having no prism at all : 

 for it will be seen by fig. 17. that the form of fig. 15. is merely 

 the result of a decrement on the edges of an octohedron formed by 

 the meeting of its two pyramids base to base ; which octohedron is 

 given by itself fig. 18. as the primitive form of the oxyd of tin. 

 But it has not hitherto been seen unmodified ; nor has any crystal 

 been noticed approaching it more nearly than that delineated by 

 fig. 21. PI. 16.* 



* Oil the subject of (he integrant molecule I do not feel competent to say more, than 

 that it has been already shewn, by a combination of all its known cleavages, that the 

 primitive crystal is mechanically divisible into tetrahedrons; but as these tetrahedrons 

 will necessarily be irregular or rather unequal in their form, it may not be satisfactory 



