76 BOTANY 
soda and sulphate of ammonia are the chief nitrogenous 
manures. 
Phosphorus is, or used to be, the chief substance 
employed in making match-heads. It is plentiful in 
bones, and is generally supplied by the farmer to his 
plants in the form of superphosphate, basic slag, and 
pone dust. The ordinary form of phosphorus ignites 
at a very low temperature. 
Potassium, the chief constituent of all potash 
manures, abounds in the ash of many plants. Kainit 
and sulphate of potash are used to supply this element 
to agricultural crops. 
Calcium is the basis of lime, which consists of 
ealeium and oxygen. Limestone, of which marble is 
a special form, is lime combined with carbon dioxide. 
The carbon dioxide may be driven off by heating the 
stone, when lime or calcium oxide remains. 
Magnesium, better known in the form of magnesia, 
its oxide, is of less importance than any other of the 
foregoing elements. 
Iron, though only traces of it are present in 
ordinary plants, is of great importance. It is the 
most widely distributed of all the metals, and is 
abundant in practically all soils. 
Silicon combined with oxygen forms silica, found 
in nature as quartz, flint, and sand. It is the chief 
constituent of the hard outer coat of grasses and of the 
cutting edge found on the leaves of many sedges. 
Chlorine is a gas which, combined with the metal 
sodium, forms common salt. Though it is essential to 
plants of the buckwheat family, it is, generally 
speaking, of little importance to agriculture. It may 
be prepared by heating a mixture of strong hydro- 
chloric acid and manganese dioxide in a test tube. It 
acts on and destroys the lining of the air passages, 
but does serious harm only when liberated in consider- 
able quantities. 
