498 Plant Case for growing Plants 



green matter which often forms on the sides of vessels filled 

 with stagnant water, Priestley spoke with more decision regard- 

 ing the agency of light; maintaining that pure air was never 

 produced by such matter while kept in the shade, but only when 

 exposed to light; that the water which contained most fixed air 

 yielded pure air most abundantly in sunshine ; and that, by the 

 agency of the sun's rays, this fixed air might be entirely dissi- 

 pated, leaving only a residue of pure air. If when this green 

 matter was yielding pure air most abundantly in sunshine, 

 the glass vessels were removed into a dark room, or the solar 

 rays were intercepted by a covering of black wax, the process, 

 he added, ceased entirely. {Observations on Air, vol. iv. p. 337.) 

 These results were confirmed and extended by the experiments 

 of Ingenhousz, who ascertained that the air which had been 

 deteriorated by the growth of plants, in the shade or through 

 the night, recovered its former purity when exposed even for 

 an hour and a half to the agency of the morning sun. In 

 like manner, air which had been vitiated by respiration, and 

 in which the carbonic acid gas was suffered to remain, was 

 soon purified by plants in sunshine, but not when they were 

 kept in the shade. This purification, he added, was effected 

 only by the leaves and green succulent stems, and by leaves 

 even when detached from the stem and immersed in water. 

 In all his experiments, carbonic acid gas seems to have been 

 present ; and he ascribes to plants the singular power of con- 

 verting that gas into respirable air, when exposed to the sun ; not, 

 however, by any process of vegetation, but solely by the oper- 

 ation of solar light. {Experiences sur les Vegetaux, t.i. p. 263., &c.) 



In addition to these facts, M. Senebier showed that light 

 was not only necessary in this process of purification, but that it 

 acted independently of heat ; for he has seen leaves, when con- 

 fined in water charged with carbonic acid, produce oxygen gas 

 by the agency of light in winter, when the temperature was 

 many degrees below freezing. In every such case, however, 

 the oxygen is derived directly from the decomposition of car- 

 bonic acid, and is always in proportion to the existing volume 

 of that gas ; but it is never furnished by the leaves themselves, 

 independently of light. {Physiol. Veg., t. iii. p. 195.) To these 

 authors succeeded M. Theodore De Saussure, who, by nume- 

 rous experiments on plants confined in close vessels, and con- 

 ducted alternately in sunshine and in shade, by careful and 

 exact analyses of the air in its different conditions, and by 

 accurate measurements of its quantities at different periods of 

 the experiment, has removed many apparent anomalies, and 

 opened the way, as we think, to a consistent and satisfactory 

 view of the subject. 



In his experiments before referred to, and published in the 



