Attention may well be drawn to a few of the compounds of Magnesia even though their use in the garden is somewhat limited. The metal itself finds no horticultural application, but its salts, the Carbonates, Chloride, Hydroxide, and Oxide are used, though to a limited extent, for application to soils which are too hot, and generally may be regarded as having cooling properties. Magnesium Carbonate in its simplest form is at times offered as a rock for wall garden building under the name of Magnesite, or as a double carbonate with lime under the name of Dolomite, but both these rocks are unsuitable. Magnesium Chloride is found with common salt in Kainit. Magnesium Oxide is prepared by burning Magnesite, just as Calcium Oxide or Quicklime is obtained by burning limestone. The Hydroxide is not, however, prepared by adding water to the oxide, but by adding a hydroxide of another metal to a solution of some compound of Magnesia. The gardener frequently uses asbestos for lining the hotwater pipes of his greenhouse, and, if he is a smoker, he may use a meerschaum pipe. These are both compounds of Magnesia. A compound of Magnesia that may have a future before it, if it can be prepared cheaply enough, is Magnesium Nitride, which slowly gives off ammonia in contact with water. It may supersede Calcium Cyanamide (see article on Lime Compounds) in this connection on hot, dry soils. It can be prepared by heating Magnesium with Nitrogen or Ammonia, but at present it is little more than a chemical curiosity. Little is known horticulturally of the uses of Magnesium Nitrate, Phosphate, Pyrophosphate, and Sulphide. The use of these does not appear to have got beyond the experimental stage. The most important compound from the gardeners point of view is MAGNESIUM SULPHATE. This the gardener or his family will probably have taken many times as an aperient under its wellknown name of Epsom Salts, but the crude compound is very useful as a manure, especially for roses. Applied at the rate of i oz. per sq. yd. in the spring, it proves distinctly advantageous, and it is well to note that the carbonate can be used in the -same manner, though at double the rate. Magnesium Sulphate is also used as an ingredient of Tonks Manure, to the article on which our readers are referred. E. T. E.
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