“A similar process goes on in mortar: the lime takes back from the atmosphere the gas that it had lost in the heat of the lime-kiln, and little by little becomes stone again. The sand mixed with it serves to disintegrate the lime, which thus more easily absorbs the air necessary for its conversion into limestone. When the mortar has fully resumed the form of limestone the courses of masonry are so strongly bound one to another that the stones themselves sometimes break rather than give way. “What is known as fat lime is lime that develops great heat when brought into contact with water, and also increases considerably in volume, forming with the water a thick, cohesive paste. On the other hand, poor lime develops but little heat, disintegrates slowly, and increases scarcely any in volume. The first kind comes from nearly pure limestone and can be mixed with a large proportion of sand, thus making a great quantity of mortar. The second kind is obtained from limestone having various foreign substances and will admit of but a small admixture of sand, thus yielding less mortar than the other. Both have the property of hardening in the air by the absorption of carbonic acid gas which converts them into limestone. “There is a third variety of lime called hydraulic lime, which has the peculiar merit of being able to harden under water. It is made from a limestone containing a certain proportion of clay. Hydraulic mortar is used for the masonry of bridges, canals, cisterns, foundations, vaults, in fact for all stone and brick work under water or in damp soil.” CHAPTER X LIME IN AGRICULTURE “To be fertile a soil must contain limestone, sand, and clay, besides the organic substances coming from humus and fertilizers. Now it may be that nature has not endowed the soil with a sufficient quantity or with any of these three constituents. Then the character of the soil must be corrected by giving it what it lacks. That is what is called improving the land. Thus a soil that is too sandy is improved by the addition of limestone and clay; one that is too compact, too clayey, is improved by adding sand and, still more, by adding limestone. Mineral substances thus added to the soil to correct it are called correctives. These substances coöperate also in the nutrition of plants, and from this point of view may be regarded as mineral fertilizers.