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THE LADIES’ MAGAZINE OF GARDENING. 
the carbonic acid gas previously produced in it by respiration or combus¬ 
tion ; whilst, in all the experiments made on foul air by Scheele, he care¬ 
fully removed this acid gas by washing the air in milk of lime before 
placing the plants in it (On Air and Fire , p. 37, 163), a procedure, as 
will presently be shown, quite sufficient to defeat the object of purifying 
the air. 
In other experiments, made on the purification of air by the green 
matter which often forms on the sides of vessels filled with stagnant water, 
Priestley spoke with more decision regarding the agency of light; main¬ 
taining that pure air was never produced by such matter while kept in 
the shade, but only when exposed to light; that the water which con¬ 
tained 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 
dissipated, 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 cover¬ 
ing 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 converting that gas into respirable air, when exposed to the sun ; 
not, however, by any process of vegetation, but solely by the operation of 
solar light (Experiences sur les Vegetaux , t. 1, 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 confined 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 carbonic 
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. 
(Phgsiol. Veg ., t. iii. p. 195.) To these authors succeeded M. Theodore 
De Saussure, who, by numerous experiments on plants confined in close 
