CHALK, Creta, is a white earth found plentifully in Britain, France, Norway, and other parts of Europe; said to have been anciently dug chiefly in the island of Crete, and thence to have received its name of Creta. They have a very easy way of digging chalk in the county of Kent in England. It is there found on the sides of hills; and the workmen undermine it so far as appears proper; then digging a trench at the top as far distant from the edge as the undermining goes at bottom, they fill this with water, which soaks through in the space of a night, upon which the whole flake falls down at once. In other parts of the kingdom, chalk generally lies deeper, and they are forced to dig for it at considerable depths, and draw it up in buckets.
Chalk is of two kinds; hard, dry, and firm, or soft and unctuous; both of which are adapted to various purposes. The hard and dry kind is much the properest for burning into lime; but the soft and unctuous chalk is best for using as a manure for lands. Chalk, whether burnt into lime or not, is in some cases an excellent manure. Its mode of operating on the soil, is explained under the article AGRICULTURE, n° 21.
Pure chalk melts easily with alkali and flint into a transparent colourless glass. With alkaline salts it melts somewhat more difficultly; and with borax somewhat more easily than with flint or sand. It requires about half its weight of borax, and its whole weight of alkali, to fuse it. Sal mirabile, and sandiver, which do not vitrify at all with the crystalline earths, form, with half their weight of chalk, the first a yellowish black, the latter a greenish, glass. Nitre, on the other hand, one of the most active fluxes for flint, does not perfectly vitrify with chalk. This earth notably promotes the vitrification of flint; a mixture of the two requiring less alkali than either of them
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separately. If glass made from flint and alkali is further saturated with the flint, so as to be incapable of bearing any further addition of that earth without becoming opaque and milky, it will still in a strong fire take up a considerable proportion, one-third or one-fourth of its weight of chalk, without injury to its transparency; hence chalk is sometimes made use of in compositions for glass, as a part of the salt may then be spared. Chalk likewise has a great effect in melting the stony matters intermixed with metallic ores, and hence might be of use in smelting ores; as indeed limestone is used for that purpose. But it is remarkable, that chalk, when deprived of its fixed air, and converted into limestone, loses much of its disposition to vitrify. It is then found to melt very difficultly and imperfectly, and to render the glass opaque and milky.
Chalk readily imbibes water; and hence masses of it are employed for drying precipitates, lakes, earthy powders that have been levigated with water and other moist preparations. Its economical uses in cleaning and polishing metalline or glass utensils are well known. In this case it is powdered and washed from any gritty matter it may contain, and is then called whiting. In medicine it is one of the most useful absorbents, and is to be looked upon simply as such: the astringent virtues which some have attributed to it have no foundation, unless in as far as the earth is saturated with an acid, with which it composes a saline concrete manifestly sub-astringent. For the further properties of chalk, see CHEMISTRY, n° 33, 127, 191, 234, 277, 342.