134 HOW CROPS GROW. 



IODINE, Sym. I, at. u-t. 127. This interesting body is a black solid at 

 ordinary temperatures, having an odor resembling that of chlorine. 

 Gently heated, it is converted into a violet vapor. It occurs in sea- 

 weeds, and is obtained from their ashes. It gives with starch a blue or 

 purple compound, and is hence employed as a test for that substance 

 (p 49). It is analogous to chlorine in its chemical relations. It is not 

 known to occur in sensible quantity in agricultural plants, although it 

 may well exist in the grasses of salt-bogs, and in the produce of soils 

 which are manured with sea-weed. 



BROMINE and FLUORINE may also exist in very small quantity in 

 plants, but these elements require no further notice in this treatise. 



SILICON AND ITS COMPOUNDS. 



Silicon, Sym. Si, at. wt. 28. This element, in the 

 free state, is only known to the chemist. It may be pre- 

 pared in three modifications : one, a brown, powdery 

 substance ; another, resembling plumbago, and a third, 

 that occurs in crystals, having the form and nearly the 

 hardness of the diamond. 



Silicon Dioxide, Sym. Si0 2 , mo. wt. 60. This com- 

 pound, known also as Silica, is widely diffused in nature, 

 and occurs to an enormous extent in rocks and soils, both 

 in the free state and in combination with other bodies. 



Free silica exists in nearly all soils, and in many rocks, 

 especially in sandstones and granites, in the form known 

 to mineralogists as quartz. The glassy, white, or trans- 

 parent, often yellowish or red, fragments of common sand, 

 which are hard enough to scratch glass, are almost inva- 

 riably this mineral. In the purest state, it is rock-crys- 

 tal. Jasper, flint, and agate are somewhat less pure 

 silica. 



Silicates. Silica is extremely insoluble in pure water 

 and in most acids. It has, therefore, none of the sensi- 

 ble qualities of acids, but is nevertheless capable of union 

 with bases. It is slowly dissolved by strong, and espe- 

 cially by hot, solutions of potash and soda, forming sol- 

 uble silicates of the alkali metals. 



EXP. 56. Formation, of ]tot<i**i um si/lmtt'. Heat a piece of quartz or 

 flint, as large as a chestnut, as hot as possible in the fire, and quench 

 suddenly in cold water. Krlui-r it to fine powder in a porcelain mor- 

 tar, and boil it in a porcelain dish with twice its weight of caustic poV 



