The universal source of lime is chalk or limestone, which consists of calcium carbonate, and from which the carbon dioxide is driven off by burning in a kiln. Many limestones, however, are far from being pure calcium carbonate, but contain large proportions of magnesia, iron and alumina, the latter perhaps originally deposited in the form of clay with the sediment from which the stone was formed. Such clay limestones when burnt yield natural cements, like oolite and other “hydraulic” limes, which are capable of setting even under water. The presence of magnesia and clay is injurious not only by diminishing the amount of lime present, but by making the lime much more difficult to slake; and iron oxide, though quite insoluble, may become mechanically fixed in the grain of the hide, and may be the cause of subsequent stains. The burning of lime in the kiln is probably not quite so simple an operation as the equations of the text-books would suggest. By mere heating, the carbonate can, it is true, be decomposed, but to do this completely a good white heat is required, which is rarely attained in practical burning, and it is probable that at least a part of the carbon dioxide present is reduced to carbon monoxide by the combustible fuel-gases, and so separated from the lime, for which it has no affinity. Carbon monoxide is the cause of the intensely poisonous character of limekiln gases, the pure dioxide being irrespirable, but not strictly poisonous. Quicklime, CaO, on coming in contact with water, combines with it with the evolution of considerable heat, becoming slaked or converted into hydrate, Ca(OH)₂. This change takes place rapidly and easily when the lime is light and porous, such as is obtained by the burning of chalk or good limestone at a low temperature; but if it has been too intensely heated or “over-burnt,” or contains silicates or other salts which fuse at the temperature of the kiln, a compact lime is formed which slakes with difficulty and extreme slowness, thus being lost to the tanner, or leading to the still more serious result of burning holes in the hides by the heat produced by slaking in contact with them. It is stated by Le Chatelier[69] that for dense limes 24-48 hours is frequently required for complete slaking in the cold, while magnesia is still more obstinate, months being sometimes necessary for the complete hydration of hard-burnt samples; and mixtures of lime and magnesia are intermediate in their character. Slaking is greatly assisted by heat, even heavily burnt magnesia being hydrated in about six hours at 100° C. Slaking is also much more rapid in a dilute solution (2 per cent.) of calcium or magnesium chloride. From these facts it is easy to deduce the reason why a suitable quantity of water, neither too much nor too little, is desirable for the rapid and effectual slaking of lime. If too little is used, the lime is only partially slaked, and it is not easy