LECTURE X. 



Inorganic constituents of plants continued.— Magnesia, Alumina, Silica, and the Oxides of 

 Iron and Manganese. — Tabular view of the constitution of the inorganic substances de- 

 scribed. — Proportions in which these several substances are found in the plants cultivated 

 for food.— Extent to which these plants exhaust the soil of inorganic vegetable food.— State 

 in which the inorganic elements exist in plants. 



§ 1. Inorganic constituents of plants continued. 



VIII. — MAGNKSIUM, MAGNESIA, CARBONATE, SULPHATE, NITRATE, ANT> 

 ^ PHOSPHATE OF MAGNESIA, CHLORIDE OF MAGNESIUM. 



1°. Carbonate of Magnesia is a tasteless earthy compound, which in 

 some parts of the world forms rocky masses and veins of considerable 

 height and thickness. It occurs more largely, however', in connection 

 with carbonate of lime in the magnesian limestones, so well known in 

 the eastern and northern parts of England, — and in similar rocks, dis- 

 tinguished by the name of dolomites or of dolomitic limestones, in va- 

 rious countries of Europe. The pure, exceedingly light, white magne- 

 sia of the shops, is partly extracted from the magnesian limestone, and 

 partly from the mother liquor of the salt pans, which generally contains 

 much magnesia. 



When pure and dry, carbonate of magnesia consists of 43^^ of magne- 

 sia, and 51 1 of carbonic acid. It dissolves readily in diluted acids (sul- 

 phuric, muriatic, and acetic,) the carbonic acid at the same time esca- 

 ping with effervescence. 



Existing as it does in many solid rocks, this carbonate of magnesia 

 may be expected to be present in the soil, and it is found in the ashes of 

 many plants. Of the ashes of some parts of plants it constitutes one- 

 sixth of the entire weight. 



When exposed to the air in a finely divided state, it gradually absorbs 

 a quantity of moisture from the atmosphere, equal to two-thirds of its 

 own weight. In this state, it dissolves in 48 times its weight of water, 

 though, when dry, it is nearly insoluble. Like carbonate of lime it is 

 also soluble -in water impregnated with carbonic acid, but in a some- 

 what greater degree. In this state of solution it may be readily carried 

 into the roots, and be the means of supplying to the ])arts of living ve- 

 getables a portion of that magnesia which is necessary to their perfect 

 growth. 



Soils containing much of this carbonate of magnesia are said to be 

 highly absorbent of moisture, and to this cause is ascribed the coldness of 

 such soils. — [Sprengel, Chemie, I., p. 645.] This opinion is, however, 

 open to doubt. 



2°. Magnesia or Caustic Magnesia, the calcined magnesia of the 

 shops. — When the carbonate of magnesia is heated to redness in the 

 open air, it parts with its carbonic acid much more readily than lime 

 does, and is brought into the state of pure or caustic magnesia. In this 

 state it does not occur in nature, but it is occasionally met with in com- 



