If burnt lime be left exposed to the air for some time, it again gradually absorbs carbon dioxide and is reconverted into calcium carbonate. When burnt lime is sprinkled with water it takes up the latter avidly, becoming very hot and finally crumbling down to a very friable white powder, consisting of slaked or hydrated lime (calcium hydroxide, Ca(OH){2}). The considerable rise of temperature in quenching the lime is due to the chemical combination of the calcium oxide and water. Both quick and slaked lime dissolve to a certain extent in water, and impart strongly alkaline properties thereto, lime being one of the strongest of bases. On exposure to the air, the solution of quick lime in water (lime-water) quickly forms an opalescent superficial film of calcium carbonate, and in a short time no more lime is present in solution, the whole having been transformed into calcium carbonate, which settles down to the bottom of the vessel as a very fine powder. Limestone that consists entirely of calcium oxide and carbon dioxide is of rare occurrence in Nature, foreign substances being nearly always present. Since the nature of these admixtures is of the greatest importance to the colour-maker, owing to the considerable influence they exert on the suitability of the minerals for his purposes, it is necessary that these extraneous substances occurring in limestone should be more closely described. Nearly all varieties of limestone contain certain proportions of ferrous and ferric oxides. The presence of ferrous oxide, when the relative amount is but small, cannot be detected by mere inspection; and even many limestones containing really appreciable quantities of ferrous oxide are pure white in colour so long as they are in large lumps. If, however, such a limestone be reduced to powder and exposed to the air for a short time, it gradually assumes a yellow tinge, the depth of which increases with the length of exposure. The cause of this change is due to the fact that ferrous oxide has a great affinity for oxygen, by absorbing which it changes into ferric oxide. (Ferrous oxide consists of FeO, ferric oxide of Fe{2}O{3}.) Ferrous oxide and its compounds are of a pale green colour which is not very noticeable, whereas ferric oxide has a very powerful yellow colour, and consequently the limestone, when its superficial area has been greatly increased by reduction to powder, assumes the yellow tinge due to ferric oxide. A limestone exhibiting this property can evidently not be used for making white earth colours, but is, at best, only suitable for mixing with other colours. Occasionally, limestone contains varying quantities of magnesia, and when this oxide is present in large amount, changes into another mineral known as dolomite. In many places this dolomite forms large masses of rock, which, however, is not employed for making colours, owing to the yellow shade imparted by the fairly large amount of ferric oxide present. _Gypsum_ (_Alabaster_)