230 MAXUAL OF AGEICULTUEE. 



PJwsphorus does not occur free in nature, but is gene- 

 rally to be found combined with oxygen and calcium. ^Vhen 

 prepared free, it is a yellowish semi-transparent, and waxy 

 solid. It is exceedingly inflammable and oxidizable. It ignites 

 on the slightest friction, whence chemists only keep it with 

 safety under water. In the air it readily oxidizes, giving off 

 white fumes, and in the dark emitting a pale lambent light. If 

 slowly oxidized, its white fumes are the oxide Pg^s- ^T°^ ^^^ 

 ignition the resulting oxide is ^2^0' — phosphoric acid. Phos- 

 phoric acid may be considered as the most important inorganic 

 constituent of plant life. Seeds have it in larger C[uantity, as it 

 constitutes about 30 per cent, of the residual ash of grains. It 

 is equally important in animal life, being a most essential con- 

 stituent of the brain, nerves, blood, and bones. 



Sulijliur is found free in nature as yellow crystals. It 

 is found combined too, with many metals, forming sulphides, 

 — which are the ores from which the several metals are usually 

 obtained. Again, combined with oxygen in addition to the 

 metals, it forms the salts called sulphates. Sulphur, during the 

 process of ignition, produces the oxide SO2, a colourless gas, soon 

 intimating its presence, however, by inducing the sense of suffo- 

 cation. SO3 is its principal oxide ; which, combined with a 

 molecule of water, is HoSO^, the important sulphuric acid. The 

 amount of sulphur found in the ash of plants is inconsiderable, 

 — over 1 or 2 per cent. 



Iron. — Of the invaluable element iron, the appearance and 

 main properties are presumably known to all. It is rarely met 

 with naturally pure, save in the form of meteoric stones ; but in 

 the well-known form of wrought iron it is nearly c^uite pure. 

 Although for minga very small percentage of j)lant ash, it is 

 still most essential, and to animal life, as well as plant, several 

 parts of animal bodies demanding it in abundance for their con- 

 stitution. It forms three classes of oxides, the presence of two 

 of which in the soil is of moment. These are (ct) the ferrous, or 

 proto-oxide, FcgOg, and (b) the ferric, or per-oxide PcgOg. As 

 the symbol shows, the ferrous has less oxygen in combination ; 

 but when exposed to the atmosphere, it greedily absorbs the 

 additional amount of oxygen, which will constitute it the ferric 

 oxide, inasmuch as the latter, again, is easily divorced from the 

 oxygen, in contact with other combinations in the soil, iron, it 

 will be seen, discharges valuable functions there, as an oxygen 

 contributor. It is iron also which imparts the variety of colour 

 to the different classes of soil. 



Manganese, prepared free, is a reddish- white, brittle, exces- 

 sively hard metal, which the slightest exposure to the air oxidises. 

 The oxide produced is MnO. MnO^, another of its oxides, is 



