22 THE LEAF 



from the gases mixed with the water, and giving off. It 

 is even more active in separating oxygen from the air, but 

 the process is not visible to the eye, because we cannot see 

 a gas except in the form of bubbles. Water is used not 

 as an aid to the plant in the performance of its function, 

 but in order to enable us to see the result. 



23. Leaves as Purifiers of the Atmosphere. Fill two 

 tumblers with water, to expel the air, and invert in a 

 shallow dish of water, having first introduced a freshly 

 cut sprig of some healthy green plant into one of them. 

 Then by means of a bent tube blow 

 into the mouth of each tumbler 

 till all the water is expelled by the 

 impure air from the lungs. Set 

 the dish in the sunshine and leave 

 showing it, taking care that the end of the 

 how leaves pudfy the atmos- cutting is in the water of the dish. 

 After forty-eight hours remove the 



tumblers by running under the mouth of each, before lift- 

 ing' from the dish, a piece of glass well coated with vase- 

 line (lard will answer) and pressing it down tight so that no 

 air can enter. Place the tumblers in an upright position, 

 keeping them securely covered. Fasten a lighted taper 

 or match to the end of a wire, plunge it quickly first into 

 one tumbler, then into the other, and note the result. It 

 is an established fact that a light will not burn in an 

 impure atmosphere ; this is why well cleaners send down 

 a lighted candle before going into a well themselves. 

 What are we to infer from the effects observed as to the 

 action of the plant upon the atmosphere ? 



This experiment will not succeed unless performed very 

 carefully, and the air must be absolutely excluded from 

 the tumblers until the instant the taper is plunged in. 



24. Leaves as Food Makers. It thus appears that 

 plants are constantly reversing the effects of animal res- 

 piration by giving off oxygen and absorbing carbon dioxide 

 from the air. Besides acting as digestive and assimilating 



