296 ^rof' Tyndall, on the Comparative View of [June 6, 



forces is nowhere more strikingly exhibited than in the phaeno- 

 mena of crystallization. I have here a solution of sulphate of 

 soda. Prolonging the mental vision beyond the boundaries of 

 sense, we see the atoms of that liquid, like squadrons under the 

 eye of an experienced general, arranging themselves into battalions, 

 gathering round a central standard, and forming themselves into 

 solid masses, which after a time assume the visible shape of the 

 crystal which I here hold in my hand. I may, like an ignorant 

 meddler wishing to hasten matters, introduce confusion into this 

 order. I do so by plunging this glass rod into the vessel ; the 

 consequent action is not the pure expression of the crystalline 

 forces ; the atoms rush together with the confusion of an unor- 

 ganized mob, and not with the steady accuracy of a disciplined 

 host. Here also in this mass of bismuth we have an example of 

 this confused crystallization ; but in the crucible behind me a slower 

 process 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 crystallized mass 

 which has thus fer escaped the abrading and disintegrating 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 



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

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

 bonate of soda. 



