This is produced by burning "limestone" whose chief ingredient is carbonate of lime. Except in the form of marble, a limestone usually contains other substances - perhaps up to 10 per cent of silica, alumina, magnesia, etc. The process of burning drives off the carbonic acid, and leaves the protoxide of calcium. This is the lime of commerce; and to preserve it from deterioration, it must be kept dry and even protected from a free circulation of air. When exposed freely to the air for a long period, it will become air-slaked; that is, it will absorb both moisture and carbonic acid from the air, and will lose it ability to harden. The first step in using common lime is. to combine it with water, which it absorbs readily so that its volume is increased to 2 1/2 or 3 1/2 times what it was before. Its weight is at the same time increased about one-fourth; and the mass, which consisted originally of large lumps with some powder, is reduced to an unctuous mass of smooth paste. The lime is then called slaked lime, the process of slaking being accompanied by the development of great heat. The purer the lime, the greater the development of heat and the greater the expansion in volume. It is soluble in water which is not already "hard," or which does not already contain considerable lime in solution. A good lime will make a smooth paste with only a very small percentage (less than 10 per cent) of foreign matter or clinker. By such simple means a lime may be readily tested.

The hardening of common lime mortar is due to the formation of a carbonate of lime (substantially the original condition of the stone) by the absorption from the atmosphere of carbonic oxide. This will penetrate for a considerable depth in course of time; but instances are common in which masonry has been torn down after having been erected many years, and the lime mortar in the interior of the mass has been found still soft and unset, since it was hermetically cut off from the carbonic oxide of the atmosphere. For the same reason, common lime mortar will not harden under water, and therefore it is utterly useless to employ it for work under water or for large masses of masonry.

When the qualities of slaking and expansion are not realized or are obtained only very imperfectly, the lime is called lean or poor (rather than fat) land its value is less and less, until it is perhaps worthless for use in making mortar, or for any other use except as fertilizer. The cost of lime is about 60 cents per barrel of 230 pounds net.