NON-LIVING THINGS 221 



wells and deep vats. How do people test wells for damps, 

 and thus know whether it is safe to descend into them? 

 How are such collections of the gas, carbon dioxide, re- 

 moved ? % 



Place a piece of lime the size of a walnut in a quart of 



water. Twenty-four hours later, pour off the clear water, 



taking care not to disturb the lime at the 



To Prepare bottom. The lime water thus simply pre- 



Lime Water . . 



pared is as good for all purposes as is that 



bought from druggists. 



Nitrogen can be separated from the other constituents 

 of the air by burning a piece of phosphorus the size of a 



pea, instead of the candle, as in the first 

 Nitrogen 



experiment with air. The dense white gas 



formed is phosphoric oxide. It is rapidly absorbed by the 

 water, leaving nearly pure nitrogen. Care should be used 

 in handling the phosphorus, and the gas should not be 

 allowed to escape into the room. 



Experiments in combustion may either precede or follow 

 or be given with the study of air, as the subjects are closely 



interrelated. Substances which burn with 

 Combustion . 



a flame are always those which, under cer- 

 tain conditions, give off combustible vapors or gases ; those 

 which glow by incandescence but from no flame are com- 

 monly some form of more or less pure carbon, as charcoal, 

 coke. The following experiments will serve to illustrate: 

 In different glasses place a few drops of carbon-bisul- 

 phide, gasoline, and alcohol. Cover the glasses for two or 

 three minutes to allow the different liquids to vaporize and 

 mix with air. Remove the cover and apply a red-hot wire 

 to the mouth of the glass containing the carbon-bisulphide. 



