DIFFUSION OF GASES. 51 



The funnel should be one with a short tube, or the vessel one 

 which is quite deep so that a small test tube which is filled with 

 water may in this condition be inverted over the 

 opening of the funnel tube. With this arrange- 

 ment of the experiment the gas will rise in the 

 inverted test tube, slowly displace a portion of 

 the water, and become collected in a sufficient 

 quantity to afford us a test. When a consider- 

 able quantity has accumulated in the test tube, we 

 may close the end of the tube in the water with 

 the thumb, lift it from the water and invert. Fig- 46- 



Apparatus for col- 



The gas will rise against the thumb. A dry lecting quantity of 



oxygen from elodea. 



soft pine splinter should be then lighted, and (Detmer.) 

 after it has burned a short time, extinguish the flame by blowing 

 upon it, when the still burning end of the splinter should be 

 brought to the mouth of the tube as the thumb is quickly moved 

 to one side. The glowing of the splinter shows that the gas is 

 oxygen. 



118. Oxygen given off by green land plants also. If we should extend 

 our experiments to land plants we would find that oxygen is given off by 

 them under these conditions of light. Land plants, however, will not do 

 this when they are immersed in water, but it is necessary to set up rather 

 complicated apparatus and to make analyses of the gases at the beginning 

 and at the close of the experiments. This has been done, however, in a suffi- 

 ciently large number of cases so that we know that all green plants in the 

 sunlight, if temperature and other conditions are favorable, give off oxygen. 



119. Absorption of carbon dioxide. We have next to inquire 

 where the oxygen comes from which is given off by green plants 

 when exposed to the sunlight, and also to learn something more 

 of the conditions necessary for the process. We know that 

 water which has been for some time exposed to the air and soil, 

 and has been agitated, like running water of streams, or the 

 water of springs, has mixed with it a considerable quantity of 

 oxygen and carbon dioxide. 



120. If we boil spring water or hydrant water which comes 

 from a stream containing oxygen and carbon dioxide, for about 20 



