262 Mr. William Phillips on the 



still unsuccessful. In the direction of the lesser planes (P' P') and 

 parallel with them, a cleavage is not only practicable, but may 

 readily be obtained by the assistance of a sharp penknife, when the 

 crystal is pressed on the fore finger beneath the thumb nail, which 

 is the most effectual mode I have been able to find. The crystals 

 are also divisible parallel with a section passing along the elongated 

 summit and down the centers of the planes P' P' of a crystal formed 

 like fig. 10. The search for natural joints in any other direction 

 was fruitless. 



If therefore we divide an elongated crystal (fig. 10.) in the direc- 

 tion of the dotted lines ab c de and b c d, being parallel sections in 

 the direction of its natural joints, we shall obtain a solid represented 

 by fig. 12, which occurs in nature, and greatly resembles some 

 crystals of the sulphate of barytes. If then this solid be cleaved 

 parallel with the planes P' P', we shall obtain a nucleus similar in 

 form to the dotted lines within ir, and of course to fig. 13, which, 

 though not in the same position, resembles in form, but not in 

 measurement, the primitive crystal of the sulphate of barytes 

 (fig. 2.) ; it is a right prism with rhombic terminations. Of these 

 solids obtained from amorphous specimens of the sulphate of lead, 

 I possess several, and am led to the conclusion that if we are to 

 depend on the cleavage of minerals for a knowledge of the forms 

 of their primitive crystals, this solid is that of the sulphate of lead. 



In my collection there is an amorphous specimen from the Lead 

 Hills, exhibiting natural joints parallel with all the planes of a right 

 prism with rhombic terminations. It is covered on one of its 

 larger sides by long and nearly flat crystals with diedral terminations, 

 lying on the mass with the terminations parallel with the natural 

 joints observable in it; and there is a still more perfectly cha- 

 racterized specimen in the collection of Mrs. Lowry. 



