86 Prof. Tyndall on the Cleavage of 



ganized mob, and not with the steady accuracyof a discij)lined host. 

 Here also in this mass of bismuth we have an example of this con- 

 fused crystallization ; but in the crucible behind me a slower pro- 

 cess is going on : here there is an architect at work " who makes 

 no chips, no din/' and who is now building the particles into 

 crystals, similar in shape and structure to those beautiful masses 

 which we see upon the table. By permitting alum to crystal- 

 lize in this slow way, we obtain these perfect octahedrons ; by 

 allowing carbonate of lime to crystallize, nature produces these 

 beautiful rhomboids; when silica crystallizes, we have formed 

 these hexagonal prisms capped at the ends by pyramids; by 

 allowing saltpetre to crystallize we have these prismatic masses, 

 and when carbon crystallizes, we have the diamond. If we wish 

 to obtain a perfect crystal, we must allow the molecular forces 

 free play : if the crystallizing mass be permitted to rest upon a 

 surface it will be flattened, and to prevent this a small crystal 

 must be so suspended as to be surrounded on all sides by the 

 liquid, or, if it rest upon the surface, it must be turned daily so 

 as to present all its faces in succession to the working builder. 

 In this way the scientific man nurses these children of his intel- 

 lect, watches over them with a care worthy of imitation, keeps all 

 influences away which might possibly invade the strict morality 

 of crystalline laws, and finally sees them developed into forms 

 of symmetry and beauty which richly reward the care bestowed 

 upon them*. 



In building up crystals these little atomic bricks often ar- 

 range themselves into layers which are perfectly parallel to 

 each other, and which can be separated by mechanical means; 

 this is called the cleavage of the crystal. I have here a crystal- 

 lized mass which has thus far escaped the abrading and disin- 

 tegrating forces which sooner or later determine the fate of 

 sugar-candy. If I am skilful enough I shall discover that this 

 crystal of sugar cleaves with peculiar facility in one direction. 

 Here again I have a mass of rock-salt : I lay my knife upon it 

 and with a blow cleave it in this direction ; but I find on further 

 examining this substance that it cleaves in more directions than 

 one. Laying my knife at right angles to its former position, 

 the crystal cleaves again ; and finally placing the knife at right 

 angles to the two former positions, the mass cleaves again. Thus 

 rock-salt cleaves in three directions, and the resulting solid is 

 this perfect cube, which may be broken up into any number of 

 smaller cubes. Here is a mass of Iceland spar, which also cleaves 

 in three directions, not at right angles, but oblique to each other, 



♦ To Mr. Pattinson, of the Felling Chemical Works, Newcastle-upon- 

 Tyne, I am indebted for some fine specimens of crystallized alum and 

 carbonate of soda. 



