38 Theory of Cryjiallizatlon. 



directed in the fame degree of obliquity as the trapezium 

 pfu t, and aflifted by the ftrpkes of a hammer. This attempt 

 was fruitlefs ; but having tried the fame operation towards, 

 the following edge b c , I uncovered a new trapezium fimilar 

 to the firft. The fourth edge a b refilled the inftrument like 

 the fecond ; but the next one a h yielded eafily to the me- 

 chanical divifion, and prefented a third trapezium of as fine 

 a polifh as the other two. It is almoft unneceflary to add, 

 that the fixth edge i h remained indivifible, like the fourth 

 and the fecond. 



I then proceeded to the inferior bafe d e f g k r, and obfer- 

 vation proved to me that the edges of this bafe, which admit- 

 ted fections like the preceding, were not the edges ef dr,gb> 

 correfponding with thofe 1 had found divifible towards the 

 upper part, but the intermediate edges de, vy, gf. The tra- 

 pezium Iqy v reprefents the fection made below the edge kr. 

 This fection is evidently parallel to that of the trapezium 

 pfu t, and in the like manner the four other trapeziums are 

 parallel two to two. But thefe different fections being in 

 the direction of the natural joining of the lamina;, I eafily 

 fucceeded to obtain more fections parallel to each of them, 

 while it was impofiible to divide the cryftal in any other di- 

 rection. By continuing this mechanical divifion, determined 

 by the parajlelifm above mentioned, I came to new feftions, 

 always nearer the axis of the prifm ; and when thefe fections 

 had made the remainders of the two bafes to difappear, the 

 prifm was transformed into the folid OX (fig. 2) terminated 

 by twelve pentagons, parallel two to two, of which thofe at 

 the extremities SAOIR, GIODE, BAODC, on the one fide, 

 and KNPQF, MNPXU, ZQPXY, on the other, were the 

 refults of the mechanical divifion, and had their common 

 vertices O, P, fituated in the centres of the bafes of the prifm, 

 (fg. 1 ). The fix lateral pentagons RSUXY, ZYRIG, &c. 

 (fg. 2) were the remains of the planes of the fame prifm. 



In proportion as I multiplied the fections, always parallel 

 ^0 the preceding, the lateral pentagon* decreafed in height j 



and 





