CARBONATES. 393 



(b.) By decomposing the sulphate of magnesia, by the carbonate of 

 an alkali. The sulphate, 1 part, dissolved in 2 parts of pure water ; 

 and 1 part of pearl ashes, dissolved in 4 or 5 parts of water.* 



Dr. Blacks process.-^- Mix the two solutions by violent agitation, 

 and let the mixture merely boil; add four parts of water, at 212, 

 and again agitate briskly. After repose, the magnesia subsides very 

 slowly, in the form of an impalpable powder ; next decant, and to 

 remove the sulphate of potassa, edulcorate ten or twelve times with 

 abundance of cold water, f 



Lastly, the magnesia must be gently pressed on a clean linen 

 cloth, for the filtering papers will not do, on account of the jelly- 

 like consistency, which magnesia, when wet, assumes. It is divided 

 into cubical pieces, by cutting it before it is quite dry, with a square 

 frame ; but when dried, it becomes an exceedingly light and spon- 

 gy powder, and this extreme lightness is one of the best marks of its 

 goodness.J 



In decomposing the sulphate of magnesia with the carbonate of 

 potassa,^ or other alkaline carbonates ; the precipitate does not al- 

 ways immediately appear, because the carbonate of magnesia is held 

 in solution by the carbonic acid, and is precipitated as fast as this is 

 evaporated, spontaneously or by heat, or by the air pump. 



3. PROPERTIES. 



(a.) A mild, white, friable, spongy substance, very light, but usu- 

 ally feebly cohering in a sort of cake, generally so light on account 

 of its porosity, as to float on water ; but it eventually absorbs so 

 much as to sink. 



(b.) Solubility in water ; 1 part of carbonate of magnesia, in 2493 

 parts of cold, and 9000 of hot water, presenting a singular anomaly, 

 which is doubtless owing to the additional elasticity given to the car- 

 bonic acid by the heat, and the hotter the water is, the more carbonic 

 acid is expelled, and the more insoluble the carbonate becomes. 



(c.) This solution changes the color of purple cabbage or mallows, 

 to green. 



(d.) Suffers no change from the air. 



* These are clarified by subsidence and decantation, and the Epsom salt, by agita- 

 ting in it the white of an egg, when the solution is just warm enough to produce 

 coagulation. 



t Hot water occasions a much longer suspension of the magnesia. 



t The theory which Dr. Black was establishing concerning the combined state 

 of carbonic acid, made him peculiarly nice in the preparation of magnesia. Black, 

 II, 57, and Dr. Murray's private instructions. 



As the common pearl ashes often contains silica, the magnesia may be examined 1 

 by sulphuric acid, in which, if pure, it is entirely soluble, while the silica will be 

 left behind. If the carbonate of ammonia or of soda, were employed, there would 

 be no danger of the presence of silica, and the bi-cavbonate of potassa would not coa- 

 tain it. 



50 



