Silica
2 of the 7 encyclopedias on this shelf carry an entry for Silica. Both are reproduced below, so you can see where they agree and where they differ.
The Encyclopedia of Founding (1892)
One of the most abundant substances found in nature. Silica is the chief component of a number of precious stones, of rock-crystals, agates, porphyry, granite, flints, sandstone, and sand. When perfectly pure it is a fine powder, very hard, and will wear away glass. When mixed with water it does not adhere, but falls to the bottom, leaving the water clear. It fuses in the oxyhydrogen blowpipe, and may be drawn into threads after the manner of glass. When silica is mixed with alkalies it melts at a lower temperature, and combines with them to form glass. The minerals, feldspar, mica, hornblende, serpentine, etc., which form the granitic and many other rocks are silicates of the alkalies and alkaline earths. Glass and pottery are compounds of silica with various metallic oxides. See GRANITE. Silica Bricks are made by incorporating about 50 pounds of lime-paste with a ton of crushed Dinas rock from the Swansea Valley. This rock contains about 97 per cent of silica, and the bricks produced from it are employed chiefly for the roofs and all exposed parts of the open-hearth steel-melting furnaces, and other similar purposes where the operations demand the most intense heat. See OPEN-HEARTH CAST STEEL.
Collier's New Encyclopedia (1921)
oxide of silicon. Pure silica occurs in nature, crystallized in six-sided prisms, terminated by six-sided pyramids in rock crystal and some other forms of quartz. It enters largely into the composition of agate, chalcedony, flint, opal, sandstone, felspar, and a vast number of other minerals. In a perfectly pure state it is quite transparent and colorless. Its hardness is next to that of the precious stones, and it has a sp. gr. of 2.6. Silica presents the general characters of an earthy base, but acts as an acid, forming with the bases compounds known as silicates. These are very abundant in nature; clay, felspar, mica, hornblende, and a large number of other common minerals are compounds of this description.