21 



found with it in combination with carbonic acid, as in dolo- 

 mite, magnesian limestone, serpentine, etc. ; it is also found 

 combined with silica, a variety of which is familiar as meer- 

 schaum, from which pipes are made. There are also two other 

 forms, the carbonate of magnesia, and the sulphate, which is 

 called Epsom salts. Magnesia must be present for plants to have 

 health, but in abundance often acts injuriously; combined with 

 phosphoric acid, it forms a large proportion of the ash of the 

 cereals, as wheat, barley, etc., and it occurs chiefly in the bran. 



Potash. When wood is burned, a greyish white ash is left: 

 this is chiefly carbonate of potash ; by strong firing this is con- 

 verted into the potashes of the shops, which is a mixture of 

 carbonate and caustic potash. This more purified, that is, con- 

 taining less carbonate and more caustic, is called pearlash ; 

 and when pure, caustic potash, which is-a combination of oxygen 

 and the metal potassium. The " lye " from wood ashes is a solu- 

 tion of carbonate of potash, and is used largely in washing, as it 

 softens hard water by precipitating the lime as carbonate, and 

 the potash unites with all greasy matters to form soap. The 

 potash of the soil is derived chiefly from feldspar, which con- 

 tains sixteen per cent., and which, as we have stated, is a con- 

 stituent of granite. Granite contains about one per cent, of 

 pure potash. 



Soda. This substance resembles potash, and appears to take 

 the place in marine plants that potash does in land plants; the 

 ash of sea weed consisting chiefly of carbonate of soda. In soils 

 it occurs chiefly as chloride of sodium or common salt, and is 

 found only in small quantity. 



Phosphorus. This substance is a yellowish semi-transparent 

 substance, soft as wax, and inflammable by the slightest friction ; 

 on burning, it combines with oxygen and forms phosphoric 

 acid. In combination with lime and magnesia it is found in 

 soils; and in the same combination it constitutes a large part 

 of the bones of animals. When bones are burnt a tine white 

 ash is left. This is the " bone ash " ot commerce and consists 

 of about eighty per cent, of phosphate of lime. If this ash is 

 mixed with charcoal, and heated in a retort, the phosphorus 

 distils over and is caught by drops in water. This, however, is 

 a dangerous experiment. As this substance is found in such 

 small quantity in soils, and in such large quantity in plants, it 

 is the one soonest exhausted, and, therefore, has to be replaced 

 by the superphosphate of commerce. 



Sulphur, also called brimstone, is a hard, yellow, brittle sub- 

 stance, devoid of smell or taste. It is found around volcanoes 

 mixed with earthy impurities : by melting the sulphur is freed 

 from these, and imported for the purpose of making sulphuric 

 acid. In the soil sulphur is generally found as sulphate of 

 lime. 



Chlorine and Fluorine. Little need be said of these sub- 



