without fresh Supplies of Water and Air. 499 



Annales de Chimie, 1797, this distinguished chemist found that 

 when garden peas (Pisum sativum), which had attained to the 

 height of between 3 in. and 4 in., were placed in a recipient of 

 atmospheric air, inverted in a saucer filled with water, and then 

 set aside in a room well lighted, but which did not receive the 

 direct rays of the sun, they grew well. At the end of ten days, 

 the volume of air was considerably diminished, its purity greatly 

 impaired, and it still retained ~q of carbonic acid. Plants of 

 Mentha aquatica effected similar changes in the air, whilst they 

 continued to grow in the shade : whence it is inferred that plants, 

 like animals, continually deteriorate the air, by converting its 

 oxygen into carbonic acid gas, when they vegetate in the shade ; 

 a result confirmed by many experiments long since made by 

 the author, and given to the public in the years 1807 and 1811. 



In prosecuting his experiments on vegetation under the direct 

 influence of light, M. De Saussure was led, with others, to the 

 conclusion, that, if the air which may have been deteriorated 

 by the growth of plants in the shade be exposed for a short 

 time to the sun's rays, it recovers its former purity. In his Re- 

 cherches Chimiques surla Vegetation, published in 1804, he has 

 established this position by numerous experiments on various 

 plants, as .Mentha aquatica, Z/ythrum Salicaria, Pinus sylvestris 

 genevensis, and Cactus Opuntia. These plants were confined in 

 glass vessels of atmospheric air, and kept for 18 or 20 hours in 

 the shade, or in perfect darkness ; but, early in the morning, 

 the vessels were taken out and exposed for 4 or 5 hours to a 

 bright sunshine ; after such exposure, the air was examined, and 

 was then found to have suffered no change whatever, either in 

 purity or in volume. 



By other experiments, the author next proceeds to show that, 

 though the air, when thus exposed to light, had recovered its 

 original composition, it must, during the experiments, have 

 undergone successive changes of deterioration and renewal. If a 

 substance, as moistened quicklime, which strongly attracts car- 

 bonic acid, were placed in the vessel with the growing plants, the 

 volume of air was observed to diminish, even although the ap- 

 paratus were placed in sunshine : the air, too, when analysed on 

 the fifth or sixth day of the experiment, afforded only y 1 ^, or 

 had lost five per cent of oxygen gas ; whilst similar plants, con- 

 fined in another vessel, but without lime, produced no change, 

 either in the purity or volume of their atmosphere. Now, the 

 diminution of volume, in the experiment with lime, shows that 

 there had been an attraction, and consequently a formation of 

 carbonic acid gas; for the lime, which produced the diminution, 

 acted only on that gas. The experiment, it is added, shows, 

 farther, that the formation of carbonic acid gas is necessary to 

 vegetation, even in sunshine, and that the reason why we do not 



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