246 Mr. William Phillips on the 



liant than such as are obtained by the pincers, but it appears that 

 the tendency of heat to separate the natural joints has also the effect 

 of cracking the crystal in other directions in which there is no 

 regular cleavage ; so that, if the blow of the hammer be not gentle, 

 the consequence commonly is that the crystal is reduced almost to 

 powder. 



Sulphate of Barytes. 



Fig. 2. 



In almost every flattish crystal of considerable dimension its 

 natural joints are apparent when holding it between the eye and 

 the light. If not, a slight blow will cause them to appear ; and if 

 by design or accident, the crystal fall flat upon the floor, it imme- 

 diately breaks parallel with some of the planes of the primitive form 

 and frequently even into the form itself, which is, a right prism 

 with rhombic bases. It follows that a substance whose laminas are 

 held together by so slight a crystalline polarity, may be cleaved or 

 split by various means ; the same may be observed of the fluate 

 and carbonate of lime, the crystals of which likewise generally 

 present indications of their natural joints. But the most certain 

 and least injurious mode of cleaving these substances is, by placing 

 the crystal, if it be large, on a table ; when, if the edge of a sharp 

 and strait knife be placed in the direction of the natural joints, a 

 slight blow with a hammer on the back of the knife readily separates 

 the crystal along them. If on the contrary the crystal be small, the 

 object is perhaps more easily attained by the assistance of a sharp 

 penknife, while holding the crystal between the finger and thumb, 

 more especially if it be very flat and thin ; in which case, a slight 



