162 SCIENTIFIC AMUSEMENTS. 



chloride of zinc. In this case reciprocal decomposition takes 

 place, again without the aid of a solvent. It is by actions more or 

 less of this kind that most metals are actually extracted from their 

 natural sources or " ores." 



Expt. 176. To produce Oxygen Gas by the Decomposition by 

 Heat of a Melted Solid. The substance termed potassium chlorate 

 is a crystallised solid substance at the ordinary temperature, but 

 on heating it melts and becomes a liquid. Place in a dry test- 

 tube as much of the salt as will lie on a sixpence, and gently heat 

 it in the Bunsen lamp ; at first it will quietly melt to a transparent 

 colourless liquid ; but as it gets hotter it will begin to effervesce 

 vigorously, owing to the escape of bubbles of the gas oxygen set 

 free in consequence of the chemical decomposition taking place in 

 the salt through the effect of heat. Light a wooden match or 

 spill, and blow out the flame, so that a spark still remains alight ; 

 introduce this smouldering end into the mouth of the test-tube ; 

 the oxygen will so accelerate the slow combustion of the spark as 

 to cause the wood to burst into flame. A wax taper or candle 

 with smouldering wick will generally behave in the same way. 



Expt. 177. To collect a Jar of Oxygen Gas. In the last ex- 

 periment the potassium chlorate did not begin to decompose and 

 evolve oxygen until it was heated to a temperature considerably 

 above its melting point. It is a curious fact that when certain 

 solid substances are added to powdered potassium chlorate, these 

 facilitate the decomposition to such an extent that oxygen is 

 evolved before the mass is heated sufficiently high to melt, 

 although the solid substances themselves are not affected by the 

 action; accordingly, for the sake of convenience, it is usual to 

 prepare oxygen by heating gently a mixture of powdered potassium 



chlorate with about a quarter of 

 its weight of black oxide of manga- 

 nese (otherwise termed manganese 

 dioxide). Place a tablespoonful of 

 the mixture in a small dry flask 

 provided with a tightly fitting cork, 

 through which passes a piece of 

 quill glass tubing bent as indicated 

 in fig. 69, so that the further end 

 of the tube can be made to dip into 

 a basin half full of water. As with 

 all other gases that do not dissolve 



Fig. 69. Flask and Bent Tube. vei T f f eel y in wate . r > a method of 



collecting oxygen is conveniently 

 applicable, by means of the pneumatic trough, which substantially 



