CRUST. The paste with which pies, tarts, &c., are made, or covered.
1. (Fine.) From flour, 1 lb.; sugar, 1⁄4 lb.; melted butter, 1⁄2 lb.; 3 eggs; milk, q. s. Requires little baking.
2. (Raised crust, for meat pies, &c.) As the last, but using 6 oz. of lard for the butter, and 2 instead of 3 eggs.
3. (Short.) From flour, 1 lb.; butter and sugar, of each 2 oz.; eggs, 2 in no.; made into a stiff paste.
Obs. The quality is improved if the whole or a portion of the butter is employed in the way directed under Puff paste. For further information hereon, consult the cookery books of Acton, Beeton, Rundell, and Soyer.
CRY′OLITE (3NaF1AlF3). A native double fluoride of aluminium and sodium, found in large quantities in Greenland, employed in the manufacture of alum, and also as a source of metallic aluminium.
CRYOPH′ORUS. See Refrigeration.
CRYS′TAL. A solid body, having a regular geometrical form. The plane surfaces by which a crystal is bounded are termed faces; these intersect in straight lines or edges; and these again meet in points, and form angles. The axis of a crystal is an imaginary line passing through its centre, and terminating either in the middle of two faces or of two edges, or in two angles; and axes terminating in similar parts of a crystal are named similar axes. When the axes of a crystal are properly chosen, and placed in a right position, the various faces are observed to group themselves in a regular and beautiful manner around these axes, and to be all so related to them as to compose a connected series, produced according to definite laws. The multitudinous forms of crystals have been distributed by mineralogists and chemists into six primary classes or systems, distinguishable from one another by the relative positions and lengths of the three axes about which the planes or faces are arranged; while the different figures of any particular system are distinguishable by the arrangement of the planes in respect to the axes. Thus, the cube or hexahedron, the rhombic dodecahedron, and the octahedron all belong to the regular system, which is characterised by 3 equal axes cutting one another at right angles. But in the cube each plane cuts 1 axis, and is parallel to 2 axes; in the dodecahedron each plane cuts 2 axes, and is parallel to a third; while in the octahedron each plane cuts the 3 axes. The names and definitions of the six crystalline systems are given below:—
| 1. Regular system. | The 3 axes equal and rectangular. |
| 2. Square prismatic s. 2 equal axes. | The 3 axes unequal, and rectangular. |
| 3. Right prismatic s. All unequal. | |
| 4. Rhombohedral s. | The 3 axes equal, but not rectangular. |
| 5. Oblique prismatic s. 1 axis rectangular to 2. | The 3 axes not equal, and not rectangular. |
| 6. Doubly o.p.s. None rectangular. |
CRYSTALLISATION. The act or process by which crystals are formed. The frequent reference to this subject in the pages of this work, and the constant employment of the process of crystallisation in the manufacture of salts, &c., in the laboratory, seem to point out the necessity of a few explanatory remarks thereon under this head. When fluid substances are suffered to pass with adequate slowness to the solid state, or when solutions of solids are slowly concentrated by evaporation, or the solvent powers of the menstruum, gradually lessened by cooling, the ultimate particles of matter frequently so arrange themselves as to form regular geometrical bodies, familiarly known by the name of crystals. This wonderful property, which is possessed by a great variety of substances in the mineral kingdom, and by nearly all saline bodies, is resorted to for many useful and important purposes in the chemical arts. It is by means of crystallisation that the majority of salts are obtained in a state of purity; for in the act of passing into the crystalline state, the foreign substances with which they are united are left behind in the mother-liquor.