1247. The rotation of crops is now found to be necessary only because certain crops exhaust the soil of the particular salts or other mineral matters which are required for their nourishment. It has already been stated that some plants require alkalies, others acids, and others combinations of acids with an alkaline base, &c. When the substances required by plants are found in the soil in which the plants grow, the plants are vigorous, but when the soil becomes exhausted of them, the plants become weak and sickly. Liebig illustrates this by that well-known case of fowls, which, when they can obtain no carbonate of lime in their food, lay eggs without shells. He states also, that young pigeons, when fed entirely on grains of wheat, in which phosphate of lime, the principal constituent of bones, is deficient, have their bones so thin and weak that they may be broken with the slightest touch; and children who are fed entirely on biscuits made of wheaten flour without milk are subject to what is called the rickets, a disease occasioned by weakness of the bones. Milk contains a great quantity of phosphate of lime; and thus if a cow is fed entirely on potatoes and turnips, which, though they contain phosphate of magnesia, have scarcely any traces of lime, the cow will lose daily a portion of the phosphate of lime deposited in her bones, and in time she will become so weak as to be unable to stand.