METHOD OF OBTAINING MAGNESIA.

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to the touch, and perfectly tasteless. It is very slightly so-luble in water. It absorbs carbonic acid gradually from theatmosphere. It changes very delicate blue vegetable colours togreen. Its attraction to the acids is weaker than those of thealcalies. Its salts are partially decomposed by ammonia, onepart of the magnesia being precipitated, and the other forming atriple compound. Its specific gravity is about 2.3. It is infusi-ble even by the most intense heat; but when mixed with someof the other earths it becomes fusible. It combines with sul-phur. It does not unite to phosphorus or carbon. It is not dis-solved by alcalies in the humid way. When heated strongly itbecomes phosphorescent. With the dense acids it becomesignited. With all the acids it forms salts of a bitter taste, mostlyvery soluble.

METHOD OF OBTAINING MAGNESIA.

The usual method of procuring magnesia, is to precipitate itfrom sulphate of magnesia by means of an alcali.

To effect this dissolve any quantity of sulphate of magnesia ina large quantity of distilled water and add to it gradually a solu-tion of perfectly pure carbonate of potash, or soda, till no moreprecipitate ensues. Then collect the precipitate on a cloth andboil it repeatedly in a large quantity of distilled water, till thisfluid passes perfectly tasteless. It is then to be dried, and ex-posed in a crucible to a red heat, till a sample of it, when cold,does not occasion the least effervescence with acids.

Rationale. ..In this process a double decomposition takesplace, the sulphuric acid of the sulphate of magnesia combineswith the alcali, and forms sulphate of potash ; and the carbonicacid of the alcali joins to the disengaged magnesia, and forms car-bonate of magnesia; the latter is precipitated, and the sulphate ofpotash remains in solution. On exposing the carbonate of mag-nesia to heat, the acid is expelled and the magnesia is left be-hind in a pure state. ,