Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/256

 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 immediately 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 laminæ 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