Some also show cleavage planes of two or more systems, and others show a crystalline structure comprised of several systems. Thus calcspar is in the 2nd, or hexagonal, whilst aragonite is in the 4th, the rhombic, system, yet both are the same substance, viz.:--carbonate of lime. Such a condition is called dimorphism; those minerals which crystallise in three systems are said to be trimorphous. Those in a number of systems are polymorphous, and of these sulphur may be taken as an example, since it possesses thirty or more modifications of its crystalline structure, though some authorities eliminate nearly all these, and, since it is most frequently in either the 4th (rhombic) or the 5th (monoclinic) systems, consider it as an example of dimorphism, rather than polymorphism. These varieties of cleavage affect the character, beauty and usefulness of the stone to a remarkable extent, and at the same time form a means of ready and certain identification and classification. CHAPTER V. PHYSICAL PROPERTIES. C--LIGHT. Probably the most important of the many important physical properties possessed by precious stones are those of light and its effects, for to these all known gems owe their beauty, if not actual fascination. When light strikes a cut or polished stone, one or more of the following effects are observed:--it may be transmitted through the stone, diaphaneity, as it is called; it may produce single or double refraction, or polarisation; if reflected, it may produce lustre or colour; or it may produce phosphorescence; so that light may be (1) transmitted; (2) reflected; or produce (3) phosphorescence. (1) TRANSMISSION.--In transmitted light we have, as stated above, single or double refraction, polarisation, and diaphaneity. To the quality of _refraction_ is due one of the chief charms of certain precious stones. It is not necessary to explain here what refraction is, for everyone will be familiar with the refractive property of a light-beam when passing through a medium denser than atmospheric air. It will be quite sufficient to say that all the rays are not equal in refractive power in all substances, so that the middle of the spectrum is generally selected as the mean for indexing purposes. It will be seen that the stones in the 1st, or cubic system, show single refraction, whereas those of all other systems show double refraction; thus, light, in passing through their substance, is deviated, part of it going one way, the other portion going in another direction--that is, at a slightly different angle--so that this property alone will isolate readily all gems belonging to the 1st system.