of certain Crystals. 5^ 



sixth ball similarly placed underneath the square will complete 

 the octohedral group, fig. 6. 



There is one observation with regard to these forms that 

 will appear paradoxical, namely, that a structure which in this 

 case was begun upon a square foundation, is really intrinsically 

 the same as that which is begun upon the triangular basis. 

 But if we lay the octohedral group, which consists oi six balls, 

 on one of its triangular sides, and consequently with an oppo- 

 site triangular face uppermost, the two groups, consisting of 

 three balls each, are then situated precisely as they would be 

 found in two adjacent strata of the triangular arrangement. 

 Hence in this position we may readily convert the octohedron 

 into a regular tetrahedron, by addition of four more balls, 

 (fig. 7.) One placed on the top of the three that are upper- 

 most forms the apex ; and if the triangular base, on which it 

 rests, be enlarged by addition of three more balls regularly dis- 

 iposed around it, the entire group often balls will then be found 

 to represent a regular tetrahedron. 



For the purpose of representing the acute rhomboid, two 

 balls must be applied at opposite sides of the smallest octohe- 

 dral group, as in fig. 9. And if a greater number of balls be 

 placed together, fig. 10 and 11, in the same form, then a com- 

 plete tetrahedral group may be removed from each extremity, 

 leaving a central octohedron, as may be sqqvl in fig. 11, which 

 corresponds to fig. 3. 



The passage of Dr. Hooke, from which I shall quote so 

 much as to connect the sense, is to be found at page 85 of his 

 Micrographia. 



" From this I shall proceed to a second considerable phe- 

 " noraenon, which these diamants (meaning thereby quartz 



