116 THE CULTURE OF FARM CROPS. 



YOUS substance. It is a far more abundant element in organic 

 nature than sulphur, which resembles it in many respects. 



PHOSPHORIC ACID, is the form in which this element is 

 of the greatest interest to farmers ; because of the universal 

 and most important relation which this compound bears to 

 vegetable and animal life. This acid is exceedingly sour ; 

 is readily soluble in water, and is corrcsive to vegetable and 

 animal substances. It does not exist in a free state, although 

 it is frequently mentioned as a constituent of the ash of all 

 plants ; but is always found in combination ; chiefly with 

 potash, soda, lime, and magnesia. In these forms it is uni- 

 versally diffused through nature and it is in these combina- 

 tions that it is of interest in the study of its relation to plant 

 growth. 



SULPHUR, is too well known to need any detailed de- 

 scription. It is only of interest in its combined form as sul- 

 phuric acid and this in its state of combination with other 

 substances. Alone, this acid is the most corrosive substance 

 known, dissolving or decomposing all organic and many 

 inorganic substances. When in combination with metals 

 or alkaline substances it forms sulphates. These exist 

 abundantly in nature and some of them, as sulphates of 

 potash and lime are useful to vegetation, while others, as sul- 

 phate of iron or sulphate of alumina are hurtful. 



SILICON, exists only artificially as a dark brown powder 

 prepared with great difficulty by a tedious chemical process. 

 In its oxide as 



SILICA, it is one of the most abundant substances, form- 

 ing the larger part of almost all minerals ; being almost the 

 sole constituent of the most common rocks and a part of al- 

 most every one of others. Its character is that of an acid, 

 as it combines with alkalies, and forms silicates, as silicate 

 of lime; of potash; of soda &c. It exists in the ash of all 

 plants without exception, and quite largely in many, form- 

 ing the outer coverings of the stems and seeds ; thus pro- 

 viding support for the plant, and protection for the germ, 

 or vital portion of the seed. These silicates are soluble in 

 water or are easily decomposed by water containing some 



