EARTHY SALTS.
The following are the preparations chictly in use : Murias muriate of baryte, Edin, Aqua calcis, lime water, Edin. Lund. Dubl.
Carbonas calcis prmparatus, prepared chalk, Loud. carbonate of lime, Edin Phosphas calcis, &lin. phosphate of lime, burnt hartshorn, Loud.
Carbonas magnesia, Edin. magnesia al ba, Land. Dubl. carbonate of magnesia.
Phosphas calcis, Edin. magnesia usta, Loud, burnt or calcined magnesia.
Sulphas alumine exsiccatus, Edin. dried sulphate of alumine, burnt alum, Loud.
In the Dublin process for making mag nesia, there is a mutual decomposition of the two salts employed. The potash unites itself to the sulphuric acid, while the carbonic acid combines with the mag nesia. The large quantity of water used is necessary for the solution of the sul phate of potash formed ; and the boiling is indispensably requisite for the expul sion of a portion of the carbonic acid, which retains a part of the magnesia in solution. Sulphate of potash may be ob tained from the liquor which passes through the filter, by evaporation. This is not pure, however, but mixed with un decomposed carbonate of potash ; for one hundred parts of crystallized carbonate of potash are sufficient for the decomposi tion of one hundred'and twenty-five parts of sulphate of magnesia ; and as the car bonate of potash of commerce contains a larger proportion of alkali than the crys tallized carbonate, a still less proportion should be used. From these quantities about forty-five parts of carbonate of mag nesia are obtained.
The ablutions should be made with very pure water ; for nicer purposes, distilled water may be used, and soft water is in every case necessary. Hard water for this process is peculiarly inadmissible, as the principle in waters, giving the property called hardness, is generally a salt of lime, which decomposes the carbonate of magnesia, by compound affinity, giving rise to carbonate of lime, while the mag.
nesia unites itself to the acid of the calca reous salt, by which the quantity of the carbonate is not only lessened, but is ren dered impure by the admixture of carbo nate of lime. Another source of impurity is the silica which the sub-carbonate of potash generally contains. It is most easily got rid of by exposing the alkaline solution to the air for several days before it is used. In proportion as it becomes saturated with carbonic acid, the silica is precipitated, and may be separated by filtration.
The carbonate of magnesia thus prepar ed, is a very light, white, opaque sub stance, without smell or taste, effervescing with acids. It is not, however, saturated with carbonic acid. By decomposing sul phate of magnesia by an alkaline carbo nate, without the application of heat, car bunate of magnesia is gradually deposited, in transparent, brilliant, hexagonal crys tals, terminated by an oblique hexagonal plane, and soluble in about four hundred and eighty times its weight of water. The crystallized carbonate of magnesia con sists of fifty acid, twenty-five magnesia, and twenty-five water ; the sub.carbonate consists of forty-eight acid, forty magne sia, and twelve water ; and the carbonate of commerce of thirty-four acid, forty-five magnesia, and twenty-one water. It is decomposed by all the acids, potash, so da, baryte, lime, and strontian, the sul phate, phosphate, nitrate, and muriate of alumina, and the super-phosphate of lime.