THE FOOD OF THE PLANT 



33 



3. By means of litmus-paper the acidity, alkalinity, 

 or neutrality of the gas should be tested. 



A strip of damp blue litmus-paper and one of red 

 are put into a jar of the gas in question. If the gas is 

 alkaline the red paper will be changed to blue ; if acid, 

 the blue will turn red ; if neutral, the papers will remain 

 unchanged. 



4. A lighted splint should be passed into a jar of the 

 gas to find out whether the gas burns or whether it is 

 able to support combustion. Note the brightness of 

 the combustion. 



(This test might in some cases be a dangerous one 

 to apply, but it is safe for all the gases dealt with in 

 this book.) 



5. The weight of the gas in comparison with the 

 weight of air should then be tested. 



This may be done by placing, mouth to mouth, two 

 gas-jars, one containing air and the other the gas under 

 mvestigation. Two sets of jars are arranged 

 as in Fig. 14, in which A represents air and 

 X the unknown gas. After a minute has 

 passed the contents of all the jars should 

 be tested. If x is lighter than A it will 

 then be found in both the upper jars; if 

 heavier, in both the lower ones. 



6. Finally it must be determined whether 

 or not lime-water is turned milky when 

 the gas is passed into it. 



If two unknown gases are compared and ^^ 



are found to respond in the same way to p^g -^^ 

 all these tests, they may then be regarded 

 as one and the same gas ; but if different results are 

 obtained in any of these tests, then the gases must 

 necessarily be different. 



We are now in a position to return to the study of 

 the nature and composition of air. 



If a bell-jar is placed over a lighted candle, the 

 candle Tery soon goes out. It seems, therefore, that 

 the air must have been altered in some way by the 



c 



m m 



