without fresh Supplies of Water and Air. 503 



tion and purity. In this way, the same air by changes of composi- 

 tion, like the same water by changes in its state or condition, may 

 be made to serve over and over again the purposes of vegetation. 



There is one circumstance of difference in the experiments 

 of De Saussure, as compared with those of Mr. Ward, which it 

 may be proper to notice. In the experiments of the former no 

 soil was used, but only a thin stratum of water, in which the roots 

 of the plants were immersed, covered the surface of the mer- 

 cury, over which the vessels were inverted. In the cases of 

 Mr. Ward, the plants were set in earth. Now, vegetable soil 

 is known to deteriorate the air, by forming carbonic acid with 

 its oxygen, in the same manner as plants do ; but the acid gas, 

 which may thus be produced, was found by De Saussure to be 

 decomposed by the joint agency of the plants and light, like 

 that produced by ordinary vegetation; and, consequently, the 

 air suffered no permanent injury. Indeed, an excess of car- 

 bonic acid, not exceeding ~ of the atmosphere in which plants 

 were confined, accelerated their vegetation in sunshine, by in- 

 creasing the proportion of oxygen ; whilst the smallest doses of 

 this gas proved injurious to that process in the shade. 



The foregoing facts demonstrate the power of light to decom- 

 pose carbonic acid gas in plants. This decomposition, however, 

 can be effected only by the concurring agency of the light and 

 the plant; and, whilst the acid gas is thus decomposed, the 

 plant itself acquires a tint of green ; so that the evolution of 

 oxygen gas by the plant, and the formation of its green colour, 

 always proceed together. Now, as the chromule, which imparts 

 colour to the leaf, is lodged in the cells of the parenchyme, it is in 

 those cells that we must suppose the decomposition of the acid 

 gas to be effected, and from them also the oxygen gas must 

 proceed. The mode in which this coloration is probably ac- 

 complished may receive illustration from the facts which follow. 

 The " colourable principle," or chromogen of Dr. Hope, is 

 readily extracted by water, and the colourless infusion which is 

 thus formed becomes red on the addition of an acid, and green 

 on the addition of an alkali. If a neutral salt be dissolved in 

 this infusion it still remains colourless ; but, if this salt be de- 

 composed by electrical agency, then the acid and alkaline ingre- 

 dients, being separated, at once produce their red and green 

 colours. Now, if we suppose the carbonic acid gas, which 

 enters the parenchyme of the leaves, to be attracted by, and to 

 combine with, the alkaline matter which is so abundant in those 

 organs, it may there form a neutral salt, and whilst this neutral 

 state continues the leaf will remain colourless; but if the chemical 

 rays of light, acting like electricity in the example before given, 

 decompose this carbonate, and cause the expulsion of its acid 

 ingredient, then the alkali, becoming predominant, will produce 



