THE VALENCY AND SPECIFIC HEAT OF THE ME.TALS 597 



much more easily than that of calcium salts. For example, mag- 

 nesium carbonate is totally decomposed at 170, magnesium oxide 

 being left* behind. This magnesia, or magiiesium oxide, is met with 

 both in an anhydrous and hydrated state in nature (the anhydrous 

 magnesia as the mineral periclase, MgO, and the hydrated magnesia 

 as brucite, MgH 2 O 2 ). Magnesia is a well-known medicine (calcined 

 magnesia magnesia usta). It is a white, extremely fine, and very 

 voluminous powder, of specific gravity 3-4 ; it is infusible by heat, and 

 only shrinks or shrivels in an oxyhydrogen flame. After long contact 

 the anhydrous magnesia combines with water, although very slowly, 

 forming the hydroxide Mg(HO) 2 , which, however, parts with its 

 water with grea$ ease when heated even below a red heat, and again 

 yields anhydrous magnesia. * This hydroxide is obtained directly as 

 a gelatinous amorphous substance when a soluble alkali is mixed with 

 a solution of any magnesium salt, MgCl 2 + 2KHO = Mg(HO) 2 + 2KC1. 

 This decomposition is complete, and nearly all the magnesium 

 passes into the precipitate ; and this clearly shows the almost perfect 

 insolubility of magnesia in water. Water dissolves a scarcely per- 

 ceptible quantity of magnesium hydroxide namely, one part is dis- 

 solved by 55,000 parts of water. Such a solution, however, has 

 an alkaline reaction, and gives, with a salt of phosphoric acid, a 

 precipitate of magnesium phosphate, which is still more insoluble. 

 Magnesia is not only dissolved by acids, forming salts, but it also dis- 

 places certain other bases for example, ammonia from ammonium 

 salts when boiled ; and the hydroxide also absorbs carbonic anhy- 

 dride from the air. The magnesium salts, like those of calcium, potas- 

 sium, and sodium, are colourless if they are formed from colourless 

 acids. Those which are soluble have a bitter taste, whence magnesia 

 has been termed bitter-earth. In comparison with the alkalis magnesia 

 is a feeble base, inasmuch as it forms somewhat unstable salts, easily 

 gives basic salts, forms acid salts with difficulty, and is able to give 

 double salts with the salts of the alkalisj which facts are characteristic 

 of feeble bases, as we shall see in becoming acquainted with the different 

 metals. 



The power of magnesium salts to form double and basic salts is 

 very frequently shown ia reactions, and is specially marked as re- 

 gards ammonium salts. If saturated solutions of magnesium and 

 ammonium sulphates are mixed together, a crystalline double salt 

 Kg(NH 4 ) 2 (SO 4 ) 2 ,6H 2 O, 20 is immediately precipitated. A strong 



20 Even a solution of ammonium chloride gives, this salt with magnesium sulphate. 

 Its sp. gr. is 1'72 ; 100 parts of water at dissolve 9, at 20 17'9 parts of the anhydrous 

 salt. At about 130 it loses all its water. 



