154 NATURE STUDY 



ments make it clear why the different gases in the air are thoroly 

 mingled, instead of the hea.vier settling to the bottom and the 

 lighter going to the top. 



Making Gas, 



The process of making illuminating gas may thus be 

 illustrated: 



Into a test-tube place bits of wood, shavings, or sawdust. 

 Arrange apparatus as in figure 81. A delivery- tube is fitted to 

 this, as in making oxygen. Jars for catching the gas are arranged 

 also as in the oxygen experiment. The test-tube full of wood is 

 now heated, and the gas coming away is caught. The first gas 

 coming off is heated air from the tube. Later, gas from the 

 decomposing wood will fill a small jar. This gas may be lighted, 

 and it will be seen to burn with a blue flame. It is a mixture 

 of gases. 



In the test-tube is found carbon, the charred remains of the 

 wood. The delivery-tube will be coated inside with tar. If a 

 larger apparatus be used, say an iron retort (this may be made 

 of iron gas pipe), a greater heat may be used, and coal may take 

 the place of wood, and by this means a considerable amount of 

 gas can be obtained. 



It may now be shown how, in making illuminating gas, a 

 very large retort is used; arrangements are made to separate the 

 tar and other substances from the gases; also those gases in the 

 mixture which interfere with the illuminating power; show that the 

 large iron gas tank corresponds to the jar catching gas in the 

 experiment. 



A visit to the gas works will now be of great interest, and 

 the main processes there carried out can be clearly understood. 



The Candle Flame. 



The special study of the candle flame will be best taken up 

 some time after the making of gas out of wood. The pupils are 



