Some Forms of Silica. 215 
water, or forming part of the mould into which roots of plants 
penetrate. 
Grasses and equisetums contain a large quantity of silica 
deposited in their tissues, which has all been obtained from the 
soil by the rootlets of the plants, principally by decomposing 
silicates of alumina and potash, such as clay and felspar. One 
has only to cut the rind of a common cane to be conscious that 
there is something there harder than ordinary bark or woody 
tissue. It also exists to such an extent in wheat-straw that, 
when it is burnt, the ashes, of which it is the principal con- 
stituent, retain the form of the unburned straw. Another 
member of the grass family, the bamboo, sometimes secretes a 
substance known as tabasheer. This is found in the knots of 
the bamboo, and occurs as a porcelain-like substance in a thin 
saucer-like layer. Valuable medicinal properties have been 
attributed to it, but that is probably because it is not easy to 
procure. It contains about ninety per cent. of silica, with water, 
potassium, iron, and calcium in indefinite proportions. It may 
be considered as vegetable opal. 
Diatoms are vegetable organisms which also secrete a very 
large quantity of silica in their cell-walls. From the variety and 
beauty of the markings which many of these exhibit, they are 
favourite objects with microscopists. They are minute alge 
found both in fresh and salt water, and their remains on the 
site of lakes, or on the sea-floor, form extensive accumulations. 
These beds are of considerable geological importance. There is 
a deposit now in course of formation in the Yellowstone Park 
many square miles in area, and often attaining a thickness of 
six feet. At Richmond, Virginia, beds of fossil diatoms occur 
thirty or forty feet thick. The forms found in this deposit are 
very beautiful, and have long been known. 
These infusorial earths, as they are sometimes called, have 
many useful applications. Thus, they are bad conductors of 
heat, and are used to form a coating for steam-pipes. They are 
also used to absorb nitro- glycerine in the manufacture of dyna- 
mite, and for filtering. Some varieties are known in commerce 
as tripoli, and are used for polishing metals. Kieselguhr is 
another name frequently applied to the purer varieties. 
Nor is the property of absorbing silica confined to vegetable 
organisms, for there are minute animals known as radiolaria, 
which occur in countless myriads floating in the sea, and their 
remains are deposited on the sea-floor over large areas, forming 
radiolarian ooze. They consist of a tiny mass of protoplasm, 
which secretes an exquisitely beautiful siliceous shell or test. 
They obtain their food by protruding portions of their proto- 
plasmic substance a considerable distance into the surrounding 
water. Their fossil remains occur plentifully in the‘rocks of 
