Page:The American Cyclopædia (1879) Volume V.djvu/545

 CRYSTALLOGRAPHY CSABA 541 tallized by some metamorphic process. 4. From any circumstances that favor the combination of the elements of a compound. Crystalliza- tions often take place at the moment of the com- bination. Origin of the Modifications of Crys- tals. The particular modifications of form pre- sented by the crystals of any substance some- times depend on the nature of the solution de- positing the crystals, and sometimes on wider terrestrial conditions. Common salt, crystalli- zing from pure water, almost invariably takes a cubic form ; but if boracic acid is present, the crystals are cubes with truncated angles ; or if the solution contains urea, the crystals are oc- tahedrons. Carbonate of copper, in course of deposition, has been observed to change the form of the crystals on the addition of a little ammonia, and again to a still different form on adding sulphuric acid. Sal ammoniac ordina- rily crystallizes in octahedrons ; but if urea is present, it forms cubes. A floating crystal forming in a solution has been seen to assume secondary planes on becoming attached to the sides of the vessel. There are many examples where a substance, as calcite, for a time crys- tallized under one form, and afterward began a new form around or on top of the first. At Bristol, Conn., six-sided prisms of calcite have been found surmounted by short, flattened cal- cite crystals of the variety called nail-head spar. At Wheatley's mine, Phoenixville, Pa., the same species, under the form of the sca- lenohedron, has been found covered and altered to a six-sided prism. Such facts prove some change, and probably a change in the nature of the solution supplying the carbonate of lime, the ingredient of calcite. In nature the crys- tals of a substance over a wide region are often identical in form. The calcite of the Niagara limestone at Lockpdrt, N. Y., in all cases has the form called dog-tooth spar, or the scaleno- hedron; that of Booneville, N. Y., the form of short hexagonal prisms ; that of the Rossie lead mine, a combination of other more com- plex forms. This is a general fact with regard to the crystallizations in rocks. In massive aggregate crystalline rocks there is a tendency to parallelism in the crystals, and hence at a granite quarry it is easier to split the granite in one direction than in others, owing to an approximate parallelism in the cleavage planes of the feldspar. To obtain large crystals arti- ficially from solutions, a large supply* of mate- rial is of course necessary. The most success- ful mode is to select certain of the best crystals that have begun to form, and supply them from time to time with new portions of the solution. They will thus continue to enlarge, the crystallizing material tending to aggregate about the ready formed crystals rather than commence a new crop. Cavities in rocks sometimes contain a vast amount -of large crys- tals. At Zinken in Germany, a single cavity was opened last century which afforded 1,000 cwt. of quartz crystals, one of which weighed 800 Ibs. In all such cases the supply of mate- rial was gradually introduced ; for so little silica is taken up by alkaline waters that the solution of silica filling the cavity at any one time could make but a thin lining over its interior. When water freezes, there is at first a sheet of ice made by the shooting of prisms over its surface. After this, as the cold continues, the crust in- creases in thickness by gradual additions to the under surface, thereby causing an elongation of prismatic crystallizations downward. The body of the ice is consequently columnar, al- though not distinctly so when examined in its firm state. In the melting of the ice of some lakes in spring, as has been observed at Lake Champlain, this columnar structure usually be- comes apparent ; and it is sometimes so decided, that when the ice is even a foot thick and strong enough to bear a horse and sleigh, the horse's foot will occasionally strike through, driving down a portion of a half-united columnar mass, which may rise again to refill the place as the foot is withdrawn. When in this con- dition, a gale at night sometimes leads to a dis- appearance of all the ice before morning. A fact like this illustrates what must be the con- dition of the earth's crust if it has slowly cooled from fusion. The crystallizing rock material below, as the crust slowly thickened, would not necessarily take columnar forms; but there would be some system of arrange- ment in the crystals which would be of a world- wide character ; and as the cleavable species feldspar is a universal mineral among igneous rocks, the earth's crust would derive some kind of structure a cleavage structure, it might be called from these conditions. Crys- tallization thus pervades the globe, and has had much to do in determining its grander surface features, as well as making gems, so- lidifying sedimentary strata, and furnishing material for the statuary and architect. It also affords man one of his best avenues for search- ing into nature, opening to view facts on which are based some of the profoundest laws in cohesive attraction, heat, light, and chemistry. There are two methods of applying mathema- tics to crystallography now much used. One, in which ordinary analytical geometry is em- ployed, is explained at length in the Anfangs- grunde der Krystallographie and Element e der theoretischen Krystallographie of Dr. C. F. Neumann, and is briefly presented in English in the 1st and 4th editions of Dana's "Mineralogy." The other is explained in Brooke and Miller's " Mineralogy," and also at more length and with more clearness in the " Physical Mineralogy " of Schrauf, published in German at Vienna. Another system, much inferior in beauty, is employed by French crystallographers. CSABA, a market town of Hungary, in the county of Bekes, situated in the great Hunga- rian plain beyond the Theiss, 63 m. N. of Temesvar, on the railway to Pesth ; pop. in 1870, 30,022, mostly Protestant. It carries on a considerable trade in corn, fruit, hemp, flax, cattle, and wine. Prior to 1846, in which year