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,