=10. Chlorin= occurs free in nature only in limited amounts and in volcanic vents. Its most common form is in combination with hydrogen, forming hydrochloric acid, or with the metals to form chlorids. It combines with sodium to form sodium chlorid or common salt (NaCl), which is the most abundant mineral ingredient in sea water and which can usually be detected in rain and ordinary terrestrial waters. In this form, also, it exists as extensive beds of rock salt, which is mined for commercial purposes. Chlorin is found uniformly in plants and must be regarded as an essential constituent thereof. Common salt applied to a soil modifies its power of attracting and holding water. =11. Phosphorus= never occurs in nature in a free state but exists in combination in greater or less quantities in all soils. Its combinations are also found in large deposits of minerals known as phosphorite and apatite and as so-called pebble deposit and phosphate rock. Phosphorus in some sort of combination is one of the most essential elements in animal and plant food. In animals its compounds form almost all of the mineral matter of the bones, and in plants they are the chief constituents of the ash of seeds. The mineral deposits of phosphorus, as well as bones, are chiefly tri-calcium phosphate, while the slag compound resulting from the basic treatment of iron ores rich in phosphorus is a tetra-calcium salt. The pebble deposits and some rock phosphates are supposed to be of organic origin, derived from the remains of marine, terrestrial, and aerial animals. Cereal crops remove about twenty pounds of phosphoric acid per acre from the soil annually and grass crops about twelve pounds. The total phosphoric acid removed annually by the cereal and grass crops in the United States is nearly four billion pounds. Gautier[1] calls attention to the fact that the oldest phosphates are met with in the igneous rocks such as basalt, trachyte, etc., and even in granite and gneiss. It is from these inorganic sources, therefore, that all phosphatic plant food must have been drawn. In the second order in age Gautier places the phosphates of hydro-mineral origin. This class not only embraces the crystalline apatites but also those phosphates of later formation formed from hot mineral waters in the jurassic, cretaceous, and tertiary deposits. These deposits are not directly suited to nourish plants. The third group of phosphates in order of age and assimilability embraces the true phosphorites containing generally some organic matter. They are all of organic origin. In caves where animal remains are deposited there is an accumulation of nitrates and phosphates.