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APPENDIX.

ing themselves into battalions, gathering round a central standard, andforming themselves into solid masses, which after a time assume thevisible shape of the crystal which I here hold in my hand. I may, likean ignorant meddler wishing to hasten matters, introduce confusion intothis order. I do so by plunging this glass rod into the vessel. Theconsequent action is not the pure expression of the crystalline forces;the atoms rush together with the confusion of an unorganized mob, andnot with the steady accuracy of a disciplined host. Here, also, in thismass of bismuth we have an example of this confused crystallization ;but in the crucible behind me a slower process is going on : here thereis an architect at work “ who makes no chips, no din,” and who is nowbuilding the particles into crystals, similar in shape and structure tothose beautiful masses which we see upon the table. By permittingalum to crystallize in this slow way, we obtain these perfect octahe-drons ; by allowing carbonate of lime to crystallize, nature producesthese beautiful rhomboids ; when silica crystallizes, we have formedthese hexagonal prisms capped at the ends by pyramids ; by allowingsaltpetre to crystallize, we have these prismatic masses; and whencarbon crystallizes, we have the diamond. If we wish to obtain aperfect crystal, we must allow the molecular forces free play : if thecrystallizing mass be permitted to rest upon a surface it will be flat-tened, and to prevent this a small crystal must be so suspended as tobe surrounded on all sides by the liquid, or, if it rest upon the suface,it must be turned daily so as to present all its faces in succession to theworking builder. In this way the scientific man nurses these childrenof his intellect, watches over them with a care worthy of imitation,keeps all influences away which might possibly invade the strictmorality of crystalline laws, and finally sees them developed into formsof symmetry and beauty which richly reward the care bestowed uponthehi.

In building up crystals, these little atomic bricks often arrange them-selves into layers which are perfectly parallel to each other, and whichcan be separated by mechanical means; this is called the cleavage ofthe crystal. I have here a crystallized mass which has thus far escapedthe abrading and disintegrating forces which, sooner or later, determinethe fate of sugar-candy. If I am skilful enough, I shall discover thatthis 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, andwith a blow cleave it in this direction ; but I find on further examiningthis substance that it cleaves in more directions than one. Laying myknife 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, andthe resulting solid is this perfect cube, which may be broken up intoany number of smaller cubes. Here is a mass of Iceland spar, which