30 



SMITH S INTERMEDIATE CHEMISTRY 



patients suffering from pneumonia or suffocation obtain some 

 relief by inhaling it in this form. It is also used in feeding 

 flames, instead of air, when intense heat is required (see acetylene 

 torch, p. 352, and calcium light, p. 385). 



2. Unfortunately, it is difficult to liberate oxygen from natural 

 substances. Saltpeter (potassium nitrate), for example, which 

 is found in many soils and can be dissolved out with water, gives 

 off oxygen only when raised to a bright red heat by the Bunsen 

 flame or blast lamp. But, even at this temperature, it gives 

 up only one-third of the oxygen it contains. 



3. In practice, we are compelled to use manufactured sub- 

 stances. Amongst the artificial substances are mercuric oxide, 

 expensive but historically interesting (p. 15), potassium chlorate, 

 perhaps the most convenient for laboratory use, and sodium per- 

 oxide. Potassium chlorate is a white crystalline substance 



FIG. 18 



used, on account of the oxygen it contains, in large quantities in 

 the manufacture of matches and fireworks. When heated in a 

 tube similar to that in Fig. 11, it first melts (357) and then, on 

 being more strongly heated, it effervesces and gives off a very 

 large volume of oxygen. Examination shows that the whole 

 of the oxygen it contains (39 per cent) can be driven out. The 

 white material which remains after the heating is identical with the 

 mineral sylvite. To the chemist it is known as potassium chloride. 



