This, the only known oxide of magnesium, is ordinarily prepared by the gentle and prolonged ignition of magnesium carbonate. A new process has lately been opened up, as follows: -
If we cause a solution of magnesium chloride to be absorbed by dry slaked lime, the magnesia set at liberty plays the part of a cement, and the matter may be moulded into small porous fragments. If one of these fragments is suspended in a solution of magnesium chloride, after some days the lime is entirely replaced by hydrate of magnesia. The fragment has been the seat of a double diffusion; the magnesium chloride has diffused itself from without to within, and is changed in the fragment into calcium chloride, which in turn becomes diffused from within to without. These two diffusions are simultaneous, and come to an end when all the lime has been replaced by magnesia. Here, then, is a means of reducing into a small volume a precipitate which would have occupied the entire bulk of the solution, if the fragment "of lime had been stirred up in it at first. The same phenomena are produced if a great number of such fragments are heaped up in a suitable vessel, where a solution of magnesia is made to circulate slowly from the top to the bottom. In 5 or 6 days the conversion is complete; the solution may be replaced by pure water, and the magnesia washed completely. On stirring up, it becomes a white pulp, which, if dried in the air, gives a very friable mass.
It is hydrated magnesia, which may remain for a long time exposed to the air without becoming notably carbonated. Its purity depends on that of the lime employed. In working on the large scale, the author uses a paste of lime, which he forces through a plate of metal pierced with small holes, so as to eliminate stones and unburnt pieces. If these " worms " fell upon the ground or into water, they would at once return to their pasty state. He therefore receives them in a solution of magnesium chloride, where they become at once covered with a slender coating of magnesia, which consolidates them so well that they may be heaped up to the height of 5 ft., still leaving between them the interstices needful for the circulation of the liquid. The paste of lime should contain 34 to 36 per cent, of anhydrous lime. The solution of magnesian salt should contain 25 to 40 grm. of anhydrous magnesia per litre. The laws of diffusion laid down by Graham are here at fault. The acceleration of the phenomena due to an increase of strength, is balanced by the resistance opposed by a more consistent deposit of magnesia. The presence of sodium chloride, always abundant in the water of salt marshes, is indifferent.
Soluble sulphates must be removed by adding the water from a former operation, rich in calcium chloride, and allowing the calcium sulphate to settle, after which the clear liquor is run off for treatment. (Chem. News.)