CRYSTAL. The term crystal, derived from a Greek word signifying a hard crust, or more specifically ice, was applied by the Greeks at least 400 }Lc. to a material which they supposed to be a hard, durable form of frozen water. This substance is the colorless, transparent variety of quartz still called rock crystal and this belief as to its nature lasted into the 16th century. The angular forms and the smooth, even surfaces of this substance were observed by the ancients, but were regarded either as acci dents or as shapes °pleasing to the gods.' The polyhedral solids obtained by the evaporation of solutions were also thought to be ice and therefore called crystal, although the error was earlier realized, for not only was crystallization recommended by Geber in the 8th century as a means of purification but the shapes of the crystals were to some extent recognized as characterizing the salt.
The secondary meaning of polyhedral form, solids bounded by plane faces, was therefore associated with the original meaning of frozen water and clear ice-like appearance and by a natural association of ideas other minerals, such as beryl, diamond, garnet and pyrite, which were observed to occur frequently in angular forms, were spoken of as crystal-like, or crystalline, and when, toward the close of the 18th century, the study of the shapes was first systematically undertaken, Rome de called the new science crystallography (q.v.). That is, the word crystal no longer meant the trans parent, ice-like substance, rock crystal, but an individual solid of any substance, whether transparent or opaque, provided this solid was bounded by plane faces at definite angles, and was formed as a result of the solidification of the substance.
This definition however is not fundamental and does not touch the essential nature of a crystal and while still held to by some authori ties it can only be consistently maintained by the creation of some new term to include all individuals possessing that character which dis tinguishes crystals from all other bodies, namely, homogeneity of internal structure. The limi tation to plane faced solids was due to the fact that external form was long the only character studied and even this character is a direct con sequence of a regular internal structure, and, although it is a striking proof of this, it is often dependent upon comparatively minor conditions at the time of solidification.
All individual solids formed at the solidifi cation of a substance, whether they are com pletely bounded by planes or partially bounded by planes or lack such boundaries and are of a shape determined by the space in which they formed exhibit equally well phenomena which stance, characters such as transmission of light, the conductivity of heat, the cohesion, the elasticity or the rate of solubility are always alike in parallel directions, and, generally wftk ing, unlike in directions which are not parallel. Or if alike in more than one direction these directions will (whatever the boundary of the individual) bear the same relations.
The general usage now is to include the faceless and partially faced individuals, either with or without a modifying term, under the term crystal, one suggestion being to distinguish them as anhedral or faceless crystals, and the following definition of Pock expresses this tend ency: A crystal is *a homogeneous solid body of definite chemical composition, whose physical properties are the same in parallel directions, but are generally different in directions not parallel.* The outward sign is the form, but its destruction does not rob the fragments of their perfect internal structure, whereas the most perfect model is not a crystal, because it lacks the internal physical characteristics.
A further modification of the definition will be needed if the doubly refracting liquids de scribed by Lehmann are admitted to be assem blages of liquid crystals. There is no definite limit between solid and liquid. In some of the softer solids one layer may be made to glide over another by pressure as in calcite and ice, other solid crystals are pliable, iodide of silver at 146° C. will flow like a thick liquid yet re tain many crystal properties and certain organic substances which possess the mobility of oil or water and yet show double refraction dichroism, interference figures and even poly hedral form are difficult to exclude. The def initions thus far limit crystals to solids; whether this limit will be sufficiently removed to in clude the so-called *liquid crystals' is still lb doubt.