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other colourless transparent minerals, with a perfect cleavage and pearly lustre--mica, talc, brucite, gypsum--by its greater hardness of 6-1/2-7. The specific gravity is 3.4. When heated before the blowpipe it decrepitates violently, breaking up into white pearly scales; it was because of this property that the mineral was named diaspore by R. J. Hauy in 1801, from [Greek: diaspeirein], "to scatter." The mineral occurs as an alteration product of corundum or emery, and is found in granular limestone and other crystalline rocks. Well-developed crystals are found in the emery deposits of the Urals and at Chester, Massachusetts, and in kaolin at Schemnitz in Hungary. If obtainable in large quantity it would be of economic importance as a source of alumina. (L. J. S) DIASTYLE (from Gr. [Greek: dia], through, and [Greek: stylos], column), in architecture, a term used to designate an intercolumniation of three or four diameters. DIATOMACEAE. For the knowledge we possess of these beautiful plants, so minute as to be undiscernible by our unaided vision, we are indebted to the assistance of the microscope. It was not till towards the close of the 18th century that the first known forms of this group were discovered by O. F. Muller. And so slow was the process of discovery in this field of scientific research that in the course of half a century, when Agardh published his _Systema algarum_ in 1824, only forty-nine species included under eight genera had been described. Since that time, however, with modern microscopes and microscopic methods, eminent botanists in all parts of the civilized world have studied these minute plants, with the result that the number of known genera and species has been greatly increased. Over 10,000 species of diatoms have been described, and about 1200 species and numerous varieties occur in the fresh waters and on the coasts of Great Britain and Ireland. Rabenhorst, in the index to his _Flora Europaea algarum_ (1864) enumerated about 4000 forms which had up to that time been discovered throughout the continent of Europe. [Illustration: FIG. 1. A and B, _Melosira arenaria._ C-E, _Melosira varians._ E, showing formation of auxospore.] [Illustration: FIG. 2.--_Synedra Ulna._] The diatoms are more commonly known among systematic botanists as the Bacillarieae, particularly on the continent of Europe, and although such an immense number of very diverse forms are included in it,
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