180 SCIENTIFIC AMUSEMENTS. 



a perceptible precipitate (of sulphate of barium) will become 

 visible. 



Expt. 195. To show the Decomposition of Carbonic Acid by 

 Growing Plants. The direct splitting up of carbon dioxide into 

 carbon and oxygen is not easy to effect, the very highest tempera- 

 tures attainable by the aid of the electric current being requisite ; 

 but the action may be indirectly effected by the aid of growing 

 vegetation. We have seen, in Expt. 153, that the exhaled breath 

 contains carbon dioxide, the chemical action taking place in the 

 body of an animal during living and breathing being that oxygen 

 is dissolved from the air in the lungs, and is carried by the circula- 

 tion of the blood all over the body ; in its passage this dissolved 

 oxygen oxidises certain of the constituents of the blood and tissues, 

 forming carbon dioxide, which is finally exhaled by the lung. 

 Living vegetation effects exactly the opposite kind of change ; i.e., 

 carbonic acid gas is dissolved from the air, &c., in the moisture of 

 the soil, and absorbed by the rootlets of plants, and so conveyed 

 to the leaves by the circulation of the sap ; under the influence of 

 sunshine the carbonic acid is decomposed, oxygen being evolved 

 in the free state, whilst the carbon of the carbonic acid, together 

 with the hydrogen and oxygen of the water present, are trans- 

 formed into solid " cellulose " (the woody part of plants) and 

 similar products. By charring these in closed vessels (Expt. 201), 

 charcoal is obtained, so that indirectly this charcoal and free 

 oxygen are thus procured from carbon dioxide by decomposition. 



Fill a jar or tumbler with water by immersion in a basin, raising 

 the jar bottom upwards out of the water, but so that the mouth is 

 still covered ; pass into the jar some sprigs of fresh mint, or of 

 some green water plant, and expose the whole to sunlight for some 

 hours. By and by bubbles of gas will form on the submerged 

 leaves, and will gradually collect at the top of the jar ; these are 

 nearly pure oxygen gas. If the water employed be mixed with a 

 quarter of its bulk of pure aerated water, so as to give a larger 

 supply of carbon dioxide, it will be practicable ultimately to collect 

 a sufficient quantity of oxygen to show that the gas will relight a 

 glowing match, and form an explosive mixture with twice its 

 bulk of hydrogen (Expts. 176 and 208). 



In order that vegetation may thus decompose carbonic acid, the 

 action of light is indispensable ; some plants (e.g., mushrooms) 

 will grow in the dark, but in such cases the green colouring-matter 

 known as chlorophyll (the tinting material in all green leaves, &c.) 

 is never formed ; apparently, the presence of this compound is 

 necessary for the evolution of oxygen from carbonic acid. 



Expt. 196. Action of Light on Silver Compounds. Certain 



