CHAP. II. FUNCTION OF NUTRITION. IQ3 



natural state to be eaten, are rendered useful and agree- 

 able additions to our salads. 



(1 79-) Action of Sun's Rays. Although the decom- 

 position of carbonic acid by the green parts of plants, 

 is perpetually carried on under the stimulus of diffused 

 light, and its effects may even be rendered apparent by 

 the action of lamp-light, which gives a slight tinge of 

 green to plants when grown in a cellar, yet in these cases 

 the process is carried on too slowly to allow of our col- 

 lecting the oxygen which is set free. But when plants 

 are placed in the direct rays of the sun, the action is so 

 much more rapid, that the oxygen may then be collected 

 in sufficient quantity to produce a striking result. If a 

 plant be immersed in pump water, under an inverted 

 glass jar placed in the direct light of the sun, in a 

 short time the surface of its leaves becomes covered with 

 minute bubbles, which presently collect at the top of 

 the jar, and are found to be nearly pure oxygen. When 

 boiled or distilled water is used from which all the 

 carbonic acid has been expelled, no such effect takes 

 place. But if another jar filled with carbonic acid be 

 also inverted over the same pan in which the jar con- 

 taining the plant is placed, and the surface of the 

 water in the pan protected by a coat of oil, to prevent 

 the escape of the gas as it is gradually imbibed by 

 the water, it will then be decomposed as before, and the 

 oxygen will collect in the upper part of the jar which 

 contains the plant, whilst an equal bulk of carbonic 

 acid will disappear from the other jar. It does not ap- 

 pear that the epidermis is essential to the success of 

 this experiment, and the decomposition of the carbonic 

 acid is equally effected by leaves which have been de- 

 prived of it. 



(180.) Action of Oxygen. A certain portion of free 

 oxygen is necessary for the formation of the carbonic 

 acid generated by the process of respiration ; but when 

 this carbonic acid is decomposed and the carbon fixed, 

 the same oxygen which is set free, will serve again 

 for a fresh formation of carbonic acid so long as there 

 o 



