274 NON-METALS AND THEIR COMBINATIONS. 



Magnesium sulphate, Magnesii sulphas, MgSO 4 .7H 2 O = 244.69 

 (Epsom salt), is obtained from spring waters, from the mineral 

 Kieserite, MgSO r H 2 O, and by decomposition of the native carbonate 

 by sulphuric acid : 



MgC0 3 + H 2 SO 4 = MgS0 4 + C0 2 + H 2 O. 



It forms colorless crystals, which have a cooling, saline, and bitter 

 taste, a neutral reaction, and are easily soluble in water. 



Effervescent magnesium sulphate, Magnesii sulphas efferves- 

 cens, is a granular mixture of magnesium sulphate, sodium bicarbonate, tar- 

 taric and citric acids, in proper proportions. It contains what is equal to 50 

 per cent, of crystallized magnesium sulphate, and, like all effervescent salts, 

 gives off carbonic acid when dissolved, which makes it more palatable. 



Magnesium nitride, Mg 3 N 2 , is obtained as a yellow, porous mass by 

 heating magnesium to red heat in nitrogen. With water it forms magnesium 

 hydroxide and ammonia, thus : 



Mg 3 N 2 + 6H 2 = 3Mg(OH) 2 + 2NH 3 , 



Remarks on tests for metals. Many of the tests for magnesium 

 and the metals to follow have already been before us when discuss- 

 ing the acids. They involve reactions of double decomposition, re- 

 sulting in the formation of an insoluble product. The solubilities of 

 the different classes of salts, such as chlorides, carbonates, sulphates, 

 etc., have been stated under the various acids, and the student, by 

 keeping these facts in mind, will be able to anticipate many of the 

 tests enumerated under the metals. Some of these are not distinctive 

 at all, but simply corroborative, because two or more metals may re- 

 spond to the same test. For example, to obtain a white precipitate 

 on adding a solution of sodium carbonate or phosphate to a solution 

 of a substance is no more a test for magnesium than for calcium, 

 strontium, barium, or any other metal whose carbonate or phosphate is 

 white and insoluble in water. In cases where distinctive tests are 

 lacking, a systematic procedure of elimination is followed. This is 

 known as qualitative analysis. 



The solubilities of the classes of salts and the different methods 

 of producing salts have been mentioned, and something has been 

 said in this respect about the two classes of compounds of metals 

 known as oxides and hydroxides. In regard to solubility in water, 

 the oxides and hydroxides are very much alike : that is, if a hydrox- 

 ide is soluble, the corresponding oxide is also soluble, and vice versa. 

 The hydroxides of the common metals that are soluble in water are 

 those of potassium, sodium, lithium, barium, strontium, and the hypo- 



