36 EXPERIMENTAL PLANT PHYSIOLOGY 



powder that dissolves in water, and an inactive gas in 

 which substances will not burn. The proportion of the 

 active to the inactive is one to four. 



It is not only by combustion that the active gas may 

 be taken from the air. Some substances will absorb it 

 without the application of any heat. 



Experiment 18 



Aim. — To find out whether damp iron fiHngs will 

 absorb the active gas from the air. 



Method. — Some iron filings are tied up in a piece of 

 muslin and put into a glass measuring-cylinder. (If 



a measuring-cylinder is not 

 available a test-tube can be 

 used instead.) The cylinder 

 is then filled with water and 

 inverted over a basin of 

 water. The muslin with 

 the filings will remain in 

 position if made to fit fairly 

 tightly. 



Air is then very gently 



blown into the cylinder until 



the level of the water inside 



is the same as that in the 



basin. This can be done by means of a piece of bent 



glass-tubing (Fig. 16). 



It may at first seem unnecessary to fill the cylinder 

 with water and then blow the water out again, but 

 there is no simpler way of getting the air inside at the 

 same pressure as the air surrounding the cylinder. 

 Try inverting an empty cylinder and note the result. 



Observations. — Very soon the water inside the 

 cylinder begins to rise, and continues to rise until one- 

 fifth of the cylinder is filled with water ; after this no 

 further change takes place. 



On testing the remaining gas it is found to be exactly 



Fig. 16 



