THE SILICATES. 117 



caustic alkali, as lime, in solution ; and the silica is then 

 made available as food for plants. 



The insoluble silicates of potash, lime, soda and magnesia 

 exist in many mineral substances. The transparent glassy 

 mineral known as mica, and often wrongly called "isin- 

 glass" and which is used for the windows of stoves, is a sili- 

 cate of alumina and potash, being composed of 46.3 per cent. 

 of silica; 36.8 per cent, of alumina; 9.2 per cent, of potash, 

 with a little iron ; the very common mineral, feldspar, is 

 another silicate of alumina, containing 16.95 per cent, of 

 potash: another abundant mineral, prehnite, contains 26 

 per cent, of lime in combination with silica and alumina ; 

 other similar minerals have soda instead of potash, and some 

 have magnesia in their composition. As these minerals 

 which form vast rocks, and mountain masses, are slowly de- 

 composed by the action of the atmosphere and the carbonic 

 acid contained in it and by the rains ; or are broken up by 

 the frosts of repeated winters, the debris is carried down 

 and borne to the lower grounds and forms the richest soils. 

 The glistening specks of mica which are seen so abundantly 

 in the soils over extensive areas, all tell the story of inex- 

 haustible stores of potash, and soda, held safely until the 

 slow action of the weather, the effective labors of the farmer, 

 and the chemical agency of the manures and fertilizers he 

 applies to the soil, unlock them from the close embrace of 

 the silica and release them to become aliment for the crops, 

 and bring comfort and w r ealth to mankind. 



These silicates are a subject for most interesting study, 

 and although silica is rarely considered by farmers as of any 

 value to them, it is really one of the most important of the 

 inorganic elements. But it exists so abundantly in nature, 

 and in such a readily available form, that like the air and 

 the water which come to us unbidden, this really precious 

 plant food is furnished as a free gift, without money or price 

 and is lavished most abundantly upon us, so that the farmer 

 is in no way concerned in regard to it. 



CHLOKINE, is a gas of a most pungent and offensive char- 

 acter,; of a greenish yellow color ; and is one of the elements 



