SZ ELEMENTARY GEOLOGY 
which is in the air and water, the importance of the 
element is greatly increased. 
Phosphorus (P).— Phosphorus is also rare, forming 
but .1% of the crust. With oxygen it enters into a 
group of minerals, the phosphates, of which the 
phosphate of lime (apatite) is the most common. 
That this element is frequently present, though in 
minute quantities, is shown by the fact that it 
enters into the bones of animals, and the tissues of 
many animals and plants; indeed, it is so important, 
that when man, by careless tillage, has drained the 
soils of the natural phosphates, some plants will not 
produce good crops until the need of the phosphate 
is artificially supplied. | 
Sulphur (S).—This element occurs near volcanoes, 
and elsewhere, as a pale yellow substance. It combines 
with many elements to form swlphides (see Pyrite, p. 49), 
or with oxygen (see Gypsum, p. 49), and a metal to form 
sulphates; and if one will burn a sulphur match in 
contact with a silver coin, he will bring about such a 
change, tarnishing the coin. Sulphur is found in the 
tissues of many animals and plants, and is scattered 
throughout the rocks, of which it forms about .03 %. 
Chlorine (C1). — This nearly transparent gas, with a 
greenish-yellow color, and a strong, irritating odor, 
is present mainly in the combination known as the 
chloride, of which common salt (pp. 30 and 50) is the 
