EESPIKA.TION OF PLANTS. 157 



will soon be given o£F, which are found to be pure oxygen ; and any- 

 carbonic acid in the water will be diminished in quantity. The same 

 leaves in darkness will not evolve any oxygen, light being essential for 

 the process. The brighter and longer continued the light, the more 

 oxygen is given off, and the greater the quantity of carbon added to 

 the plant. If a healthy plant is covered by a bell jar, and exposed to 

 light for twelve hours, oxygen will be formed, and if carbonic acid be 

 added to the air, it will be decomposed, and the oxygen will increase. 

 During the night the action is reversed, and if the plant is left twelve 

 hours in darkness, the oxygen wiU decrease, while carbonic acid wUl 

 increase. Daubeny, from his experiments respecting the action of 

 plants on a known amount of atmospheric air, states that leaves are 

 requisite for the purification of the air, that the action of light on them 

 gives rise to the emission of oxygen and the decomposition of carbonic 

 acid, that for the elimination of oxygen the presence of carbonic acid 

 is requisite, and that the greatest amount of oxygen which can, by 

 vegetable respiration, be added to air confined within a jar is 18 per 

 cent. The following is a simple experiment showing the production of 

 oxygen by green leaves under the action of light. If a green leaf is 

 placed in an atmosphere composed of hydrogen and carbonic acid, and 

 a stick of phosphorus is introduced, no apparent action takes place 

 in the dark, but the moment a beam of light, or the electric light 

 rays, are thrown on it, white fumes of phosphorous anhydride are 

 instantly produced, indicating the combination of the free oxygen, 

 evolved from the leaf under the action of light, with the phosphorus. 



The following are the results of Boussingault's experiments on the 

 functions of leaves : — 



1. The volume of COj decomposed, is identical with that of the oxygen produced. 



2. Leaves decompose pwe carbonic acid with extreme slowness. 



3. Leaves in presence of ordinary air and COj effect readily the decomposition of 



the latter. 



4. Leaves decompose COj in sunlight, when it is diluted with hydrogen, nitrogen, 



carbonic oxide, or marsh gas. 



5. Leaves lose the power of decomposing carbonic acid as they lose water (becoming 



dry). 



6. The upper surface of thick leaves, such as those of the Cherry Laurel, decom- 



pose more CO^ than the under, in the proportion of 4 to 1 in the sun ; 

 whereas in the shade it is as 2 to 1. Leaves having a thin parenchyma do 

 not differ in' the power of decomposing in the upper or under surface. 



The fixation of carbon probably takes place gradually, giving rise, 

 at different stages, to the formation of various organic compounds. 

 Thus, two molecules of carbonic acid, by losing one atom of oxygen, 

 become oxalic acid ; this oxalic acid, with the aid of water, may yield 

 other acids, from which, by the elimination of oxygen and the addition 

 of the elements of water, various unazotised matters, as starch, gum, 

 and sugar, may be derived; these changes being promoted by the 



