THE LEAF 



169 



a glass vessel full of water, and invert over it a glass funnel, or a long- 

 necked bottle from which the bottom has been removed as directed in Exp. 

 53. Expel the air from the neck of the funnel — 

 or bottle — by submerging and corking under water 

 so as to make it air-tight. Place in the sunlight and 

 notice the bubbles that begin to rise from the cut 

 end of the plant. When they have partly filled the 

 neck of the funnel, remove the stopper and thrust 

 in a glowing splinter. If it bursts into flame, or 

 glows more brightly, what is the gas that was given 

 off? (Exp. 22.) 



As oxygen is not a product of respiration, some 

 other process must be at work here, during which 

 oxygen is set free, and some other substance used 

 up. (Exps. 24 and 25.) 



Experiment 67. What is the substance taken 



IN WHEN OXYGEN IS GIVEN OFF ? — Fill twO glaSS 



jars, or 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 it, taking care that the end of the cutting is in 



the water of the dish. After forty-eight hours re- oxygen in sunlight 



move the tumblers by running under the mouth of 



each, before lifting from the dish, a piece of glass well coated with vaseline 



(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. What 

 was the gas blown from your lungs into the 

 Fig. 226. — Experiment jars? (Exps. 23, 24.) Why did the taper not 



for showing that leaves absorb go out in the second jar? What had become 



carbon dioxide from the at- ^ ,, , j- • i o 



mosphere. O^ <^he carbon dioxide ? 



Experiment 68. To show that light 



IS NECESSARY FOR A PLANT TO ABSORB CARBON DIOXIDE AND GIVE OFF 



OXYGEN. — Repeat Exp. 66, keeping the plant in a dark or shady place; 

 do you see any bubbles? Test with a glowing match; is any oxygen 



Fig. 225. — Experi- 

 ment showing that 

 green plants give off 



