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, No. 20, 25, &c.

Pure chalk melts easily with alkali and flint into a transparent colourless glass. With alkaline salts it melts somewhat more difficulty, 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 sand-ver, 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 separately. If glass made from flint and alkali is further ther 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 glaas, 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 glaas 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 metallic or glaas utensils are well known. In this case it is powdered and washed from any gritty matter it may contain, and is then called whitings.—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 so 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, Index.

Black Chalk, a name given by painters to a species of earth with which they draw on blue paper, &c. It is found in pieces from two to ten feet long, and from four inches to twenty in breadth, generally flat, but somewhat rising in the middle, and thinner towards the edges, commonly lying in large quantities together. While in the earth, it is mouldy and flaky; but being dried, it becomes considerably hard and very light; but always breaks in some particular direction; and if attentively examined when fresh broken, appears of a striated texture. To the touch it is soft and smooth, stains very freely, and by virtue of its smoothness makes very neat marks. It is easily reduced into an impalpable soft powder without any diminution of its blackness. In this state it mixes easily with oil into a smooth paste; and being diffused through water, it slowly settles in a black slimy or muddy form; proprieties which make its use very convenient to the painters both in oil and water colours. It appears to be an earth quite different from common chalk, and rather of the flaky bituminous kind. In the fire it becomes white with a reddish cast, and very friable, retaining its flaky structure, and looking much like the white flaky masses which some forts of pit-coal leave in burning. Neither the chalk nor these ashes are at all affected by acids.

The colour-shops are supplied with this earth from Italy or Germany; though some parts of England afford substances nearly, if not entirely, of the same quality, and which are found to be equally serviceable both for marking and as black paints. Such particularly is the black earth called kilow, said by Dr Merret in his Pinax Rerum Britannicamnum to be found in Lancashire; and by Mr Da Cotta, in his history of fossils, to be plentiful near the top of Cay-Avon, an high hill in Merionethshire.

Red Chalk, an earth much used by painters and artists, and common in the colour-shops. It is properly an indurated clayey ochre; and is dug in Germany, Italy, Spain, and France, but in greatest quantity in Flanders. It is of a fine, even, and firm texture; very heavy, and very hard; of a pale red on the outside, but of a deep dusky chocolate colour within. It adheres firmly to the tongue, is perfectly infusible to the taste, and makes no effervescence with acids.

Chalk-Land. Barley and wheat will succeed very well on the better sort of chalky land, and oats generally do well on any kind of it. The natural produce of this sort of land in weeds, is that sort of small vetch called the time-tare, with poppies, may-weed, &c. Sainfoin and hop-clover will generally succeed tolerably well on these lands; and, where they are of the better sort, the great clover will do. The best manure is dung, old rags, and the sheep-dung left after folding them.

Chalk-Stones, in medicine, signify the concretions of calcareous matter in the hands and feet of people violently afflicted with the gout. Leeuwenhoek has been at the pains of examining these by the microscope. He divides them into three parts. The first is composed of various small parcels of matter looking like white grains of sand; this is harder and drier, and also whiter, than the rest. When examined with large magnifiers, these are found to be composed of oblong particles laid closely and evenly together; though the whole small stones are opaque, these component parts of them are pellucid, and resemble pieces of horse-hair cut short, only that they are somewhat pointed at both ends. These are so extremely thin, that Mr Leeuwenhoek computes that 1000 of them placed together would not amount to the size of one hair of our heads. The whole stones in this harder part of the chalk are not composed of these particles, but there are confusedly thrown in among them some broken parts of other substances, and in a few places some globules of blood and small remains of other juices. The second kind of chalky matter is less hard and less white than the former, and is composed of fragments or irregular parts of those oblong bodies which compose the first or hardest kind, and these are mixed among tough and clear matter, interspersed with the small broken globules of blood discoverable in the former, but in much greater quantity. The third kind appears red to the naked eye; and, when examined with glaases, is found to be a more tough and clammy white matter, in which a great number of globules of blood are interspersed; these give it the red appearance it has.