Measurement and Cleavage of certain Primitive Crystals. 245 



more readily obtained than the second, but it is extremely difficult 

 to effect it parallel with the primitive planes. Yet having sacrificed 

 very many crystals in the pursuit of this object, I possess several 

 fragments in each direction, having brilliant surfaces. The hardness, 

 and at the same time, the brittleness of the substance were obstacles 

 to the discovery of its natural joints, which it was difficult to find 

 the means of overcoming ; but, after resorting to various expedients, 

 I found the employment of a pair of steel pincers the most certain 

 mode of effecting it. Being sharp, with edges about an inch long, 

 they seize on the square prism, equally throughout its whole length ; 

 and, if the edges be carefully placed along the center of the prism, or 

 parallel thereto, it splits, by considerable pressure, shewing plane* 

 parallel to two of the four sides of the prism. By the same means, 

 it may again be split at right angles. In the endeavour to obtain 

 cleavages in the directions of the diagonals of the prism, I found it 

 impossible to lay hold of the sharp edges lengthwise, because tha 

 edges of the instrument were also sharp ; and therefore took off the 

 pyramid as nearly at right angles with the prism as possible, pro- 

 ducing a plane, parallel to which it was requisite to procure another 

 at the other end of the prism : applying then the pincers to 

 these parallel planes, and in the direction of the diagonal, the prism 

 sometimes yielded in that direction. But the crystals of tin are 

 most readily split according to their natural joints, by placing them 

 for some time in a common fire or a smith's forge ; a slight blow 

 with a hammer afterwards reduces them into numerous small pieces, 

 and amongst such, I have found fragments in all the directions 

 above mentioned ; indeed this was the only means I could devise 

 for obtaining them parallel with the faces of the primitive octohe- 

 dron. The subjecting of the crystals to the action of heat, has 

 however some disadvantages : not only are the fragments less bril- 

 Vol. iv. 2 i 



