OXYGEN AND ITS SALINK COMBINATIONS 



159 



passed through a Woulfe's bottle containing a solution of caustic potash, 

 to purify it from carl ionic anhydride and chlorine, which accompany the 

 evolution of oxygen from commercial manganese peroxide, and the ua- is 

 not collected until a thin smouldering taper placed in front of the escape 

 orifice bursts into flame, which shows that the gas coming off is oxygen. 

 By this method of decomposition of the manganese peroxide by sul- 



FIG. 28. Preparation of oxygen from manganese peroxide and sulphuric' acid. The gas evolved 

 is passed through a Woulfe's bottle containing caustic potash. 



phuric acid there is evolved, not, as in heating, one-third, but one-half 

 of the oxygen contained in the peroxide (Mn0 2 + H 2 S0 4 = MnS0 4 -f 

 HoO + O) that is, from 50 grams of peroxide about 7i grams, or 

 about 5^ litres, of oxygen, 10 whilst by heating only about 3^ litres are 

 obtained. The chemists of Lavoisier's time generally obtained oxygen 

 by heating manganese peroxide. Now there are more convenient 

 methods known. 



3. A third source to which recourse may be had for obtaining 

 oxygen is represented in acids and salts containing much oxygen, and 

 which are capable, by parting with a portion or all of their oxygen, 

 of being converted into other compounds (lower products of oxida- 

 tion) which are more difficultly decomposed. These acids and salts 

 (like peroxides) evolve oxygen either on heating alone, or when 

 heated with some other substance. Sulphuric acid may be taken 

 as an example of an acid which is decomposed by the action of heat 

 alone, 11 for it breaks up at a red heat into water, sulphurous anhydride, 



10 Scheele, in 1785, discovered the method of obtaining oxygen by treating manganese 

 peroxide with sulphuric acid. 



11 All acids rich in oxygen, and especially those whose elements form lower oxides, 

 evolve oxygen either directly at the ordinary temperature (for instance, ferric acid), or on 

 hciiting (for instance, nitric, manganic, chromic, chloric, and others), or if basic lou.-r 

 oxides are formed from them, by heating with sulphuric acid. Thus the salts 



