540 CRYSTALLOGRAPHY the twins consist of five united crystals. In compound crystals of another kind, the compo- sition is produced after the crystal has begun to form, instead of in the first or nucleal parti- cle. A prism, as in rutile, after elongating for a while, takes a sudden bend at each extremity at a particular angle, depending on the values of the axes. In another case, as albite, which is triclinic, a flat prism begins as a thin plate ; then a reversed layer is added to either sur- face; then another like the first plate; then another reversed ; and so on, until the crystal consists of a large number of lamellae, the al- ternate of them reversed in position, yet all as solidly united as if a simple crystal. Such a kind of composition may be indicated on the surface in a series of fine striations or furrows,' each due to a new plane of composition ; and they are frequently so fine as to be detected only by means of a magnifying glass. This mode of twin is additional proof of the polar- ity of the crystallogenic molecule. If there were not some inherent difference in the ex- tremities or opposite sides of the molecules or their axes, which is equivalent to polarity, there could not be this series of reversions during the formation of the crystal. External electric or other influence may be the cause of the rever- sion, XL While simple and twin crystals form when circumstances are favorable, in other cases the solidifying material becomes an aggregate of crystalline particles. Regular crystals often require for their formation the nicest adjust- ment of circumstances as to supply of material, temperature, rate of cooling, or evaporation, &c. ; and hence imperfect crystallizations are far the most common in nature. A weak so- lution spread over a surface may produce a de- posit of minute crystals, which, if the solution continues to be gradually supplied, will slowly lengthen, and produce a fibrous or columnar structure. In other cases, whether crystalliza- tion take place from solution, or fusion, or oth- erwise, the result is only a confused aggregate of grains, or the granular structure. Under these circumstances, the tendency in force to exert influence radially from any centre where it is developed or begins action, often leads to concentric or radiated aggregations, or concre- tions. The point which first commences to solidify, or else a foreign body, as a fragment of wood or a shell, becomes such a centre ; and aggregation goes on around it, until the con- cretion has reached its limits. Basalt and trap rocks which have been formed from fusion are often divided into columns, and the columns have concave and convex surfaces at the joints or cross fractures, proving that they are con- cretionary in origin. The centre or axis of each column is the centre of the concretionary structure, and therefore it was the position of the first solidifying points in the cooling mass. The distance therefore between the initial so- lidifying points determines in any case the size of the columns; and as the columns are large the thicker the cooling mass, the distance is greater the slower the cooling. The cracks separating the columns are supposed to be owing to contraction on cooling. XII. The system of crystallization of a given substance sometimes undergoes a total change, owing to external causes. Carbonate of lime ordinarily crystallizes in rhombohedrons, and is then called calcite ; but in certain cases it crystal- lizes in trimetric prisms, and it is then called aragonite. The aragonite appears to form when the solution has a higher than the ordi- nary temperature. This property of present- ing two independent forms is called dimor- phism. Besides difference of form, there is in all such cases a difference of hardness and specific gravity. Carbon crystallizes in one set of forms, which are isometric, in the dia- mond, and in another, hexagonal, in gra- phite. Glass and stone are dimorphous states of the same substance, and the former may be changed into the latter by slow cooling. Modes of Crystallization. Crystallization re-' quires freedom of movement among the par- ticles engaged in the process. It may take place: 1. From solution, where a solvent serves to disunite the molecules of a solid, and give them the free movement required. The crystallization of sugar or alum from a concen- trated solution is an example of this method. The alum solution is simply set away to cool, and the crystals slowly form and cover any ob- ject that may be placed in the solution. With many solutions evaporation cautiously carried on will throw down a crop of crystals. Sea water, on slow evaporation, first deposits gyp- sum, afterward common salt, and then its mag- nesias salts. 2. From a state of fusion or of vapor. Heat in this case is the dissevering agent, and the removal of heat permits resolid- ification. Thus water becomes ice, and aque- ous vapor snow ; and melted lead, sulphur, and other substances may come out in perfect crys- tals. If a mass of melted sulphur, or of bis- muth, after it has crusted over, be tapped and the interior run out, the cavity within will be found lined with crystals. Camphor, when sublimed by a gentle heat, condenses again in delicate crystallizations. 3. From long con- tinued heat without fusion. The heat used for tempering steel is far short of fusion, and yet it allows of a change in the size of the grains throughout the mass. Heat has crystallized beds of earthy sediment, and thus changed them into gneiss and mica schist without fusing the rocks ; and there is reason to believe that even a low degree of heat long continued is sufficient for these results. By this means statuary marble, one of the earth's crystalline rocks, has been f made of fossiliferous limestones. The white marble of Berkshire, Mass., is probably of the same formation with either the Chazy or the Trenton limestone, rocks full of fossils, in cen- tral New York and elsewhere. Such altered rocks are termed in geology metamorphic rocks. Nearly all the gems, and far the larger part of the crystalline rocks of the world, were crys-
