EPSOM-SALT AND GLAUBER'S SALT. 21 



7. CAKBONATE OF POTASSA AND MAGNESIA.* 

 KO, C0 2 ; 2(MgO, C0 2 ) + 9 HO." 



When ignited, this salt loses three-fourths of its car- 

 bonic acid, and the whole of its water, leaving a mix- 

 ture of carbonate of potassa and magnesia, which may 

 be separated by water, and quantitatively determined, 

 the potassa for this purpose being converted into chlo- 

 ride of potassium. The magnesia should not be washed 

 longer than is necessary, since it is not entirely insolu- 

 ble in water. (Magnesia is more soluble in hot water, 

 and on this account cold is preferable.) 



The total quantity of carbonic acid contained in the 

 salt is determined by expelling it in an apparatus 

 arranged for the quantitative determination of carbonic 

 acid. The amount of water may then be inferred by 

 difference. 



The joint weight of the water and carbonic acid may 

 be determined by fusing a quantity of vitrified borax 

 in a platinum crucible, weighing when cool, intro- 

 ducing the salt, again weighing, and fusing over the 

 gas flame until all the carbonic acid is evolved, and 

 the fused borax becomes clear. The loss of weight 

 expresses the joint amount of water and carbonic acid. 



8. EPSOM-SALT AND GLAUBER'S-SALT. 

 MgO, S0 3 +7 HO and NaO, SO 3 + 10 HO. 



One hundred parts of pure sulphate of magnesia 

 give, by the method described in No. 6, 45.12 parts of 

 phosphate of magnesia. A specimen of Epsom-salt 



* Obtained in crystals on mixing a solution of chloride of magne- 

 sium with a warm saturated solution of bicarbonate of potassa. 



