Minor Constituents of Air 



243 



and, in time, sufficient of this gas will collect in the 



upper end of the test-tube for its 



nature to be tested. Remove the 



tube, placing the thumb under its 



mouth to prevent the entrance of 



air, invert it, and apply a glowing 



splinter of wood to the mouth of 



the tube. The wood will burst into 



flame a result which we learnt in 



Experiment 44 to be characteristic 



of the gas oxygen. 



The plant has acted upon the 

 carbon dioxide dissolved in the tap- 

 water and has produced oxygen as 

 a waste-product. To emphasise the 

 part played by the dissolved carbon 

 dioxide the experiment may be re- 

 peated, the flask being filled with 

 (a) tap -water which has been boiled 

 for twenty minutes and then quickly 

 cooled in a flask of which the mouth 

 is lightly stoppered with a plug of 

 cotton- wool ; (6) some water through which carbon 

 dioxide has been allowed to bubble for several minutes. 



Exercise for Student. 



Account for the varying behaviour of the water-weed under the 

 different conditions suggested above. 



165. It must not be inferred that plants under all 

 circumstances evolve oxygen. Carbon dioxide is pro- 

 duced by the activity of a living plant in certain 

 conditions. As an example of this, the following 

 experiment is instructive. 



162 



