158 SCIENTIFIC AGRICULTURE. 



water slowly, and allows it to escape easily. It is neither dis- 

 solved nor acted upon by any acid except the fluoric, with 

 which it unites find forms a fluoride of silicon. 



The equivalent number of silicon is 22.22, its specific 

 gravity 2.GG. The fixed alkalies easily unite with silicon, and 

 form silicates, as the silicate of potash, lime, <fcc. It forms an 

 important ingredient in porcelain, glass, and the enamel or 

 glazing of stone ware. The salts of silicon are not numerous; 

 they are all insoluble in water, (according to Prof. Johnston,) 

 except those of potash and soda. 



As silicon is so important an element in plants, and so inva- 

 riably present in all productive soils, a knowledge of its chemi- 

 cal character, and the best means of rendering it available to 

 the roots of growing vegetation, is indispensable. 



ALUMINUM. 



This metallic earth is found in greater abundance in nature 

 than lime, being one of the principal ingredients in nearly all 

 rocks, except the purest limestone : it is the principal element 

 of clay, and exists largely in garnet, albite and mica: it is found 

 also in the ashes of most plants. In its native state it is usually 

 found in combination with silica, and sometimes with sulphuric 

 or phosphoric acid: it is also found nearly pure, or uncombined, 

 in the ruby and sapphire, two beautiful precious stones. Alu- 

 mina is an oxide, and the only one known of the earth alumi- 

 num ; it is white, tasteless and inodorous ; its equivalent num- 

 ber is 13.7. 



It dissolves in acids and in solutions of caustic alkalies; it 

 has a strong tendency to unite with organic matters, and has 

 fcbo a greater affinity for water than any of the other elemen- 

 tary earths. "When mixed with silica in the proportion to 

 form clay, it is easily molded into any form, as in stone and 

 earthen ware: it loses part of its tenacity by fire,. hence the 

 benefit of burning clay soils. Alum is a salt formed by the 

 union of potash, alumina and sulphuiic acid, this salt is 



