Mr. William Phillips on the OxydofTin 355 



and in some instances I obtained perfect co-incidences by the reflect- . 

 ing goniometer between the faces P and a fig. 9. and those of 

 P and dy fig. 11. having eight fractures of the figs. 4. and 9. which 

 permit the use of that goniometer. 



On three fragments similar to fig. 9. I have attempted a mecha- 

 ni^cal division in the direction of the small planes a f e fig. 10. 

 and have succeeded in obtaining one or the other on each, so as to 

 warrant the conclusion of the practicability of the whole. It will 

 be noticed that if a division were still pursued in the direction of 

 the planes afe^ the consequence would be that the planes on the 

 summit of fig. 10 would become in form similar to those of 

 PPfig. 11. 



The cleavages obtained by the planes a f e demonstrate the 

 possibility of a mechanical division parallel with each terminal face 

 of a crystal similar to fig. 11. The probability of this in two out 

 of the four directions may be argued from what is known respect- 

 ing the formation of the macles of this substance. 



It was long since determined by L'hermina that the common 

 made represented by fig. 186. PI. 24. as will hereafter be further 

 noticed, is the result of a section of a common prismatic crystal, 

 fig. 27. PI. 16. in a direction parallel with one or the other of the 

 edges of its pyramid. The planes forming the pyramid of fig. 11. 

 PI. 15. are usually considered to be the effect of a decrement on 

 those edges, but the reverse is the fact ; for, by figs. 18, 19, 20, 21, 

 25,26,27, it will be seen that the pyramid of fig. 27. is the result of 

 a modification of the primitive crystal described by the planes 2,2 on 

 fig. 22; and it will be equally obvious that if the section determined by 

 L'hermina be parallel with one or other of the edges of the pyramids 

 of fig. 27. it must also be parallel with one or the other of the planes 

 of the primitive octohedron, consequently with one or the other of 



2 y 2 



