or the volume of the air* In the same manner, the 
atmospheric air, in which two plants ot men ; net aqua- 
tica had grown for ten days, and been exposed six 
hours each day to the rays of the sun, experienced 
no change either in its purity or its volume. The 
ly thrum salicaria^ vegetating for seven days, under a 
similar exposure to the sun's rays, produced no change 
in the volume or composition of its atmosphere; and 
M. de Saussure adds, that he could not be mistaken 
in the measurement of the volume of air more than 
one-fourth of a cubic inch. The />htu.s gcttcrt t;si 
and cactus opuntia^ placed in the same circumstances, 
were likewise found to produce no sensible change 
in the purity or volume of their atmosphere *. Hence 
we learn, that, when plants are confined' in given 
portions of air, and placed alternately, for several 
days, in sunshine and in the shade, they neither vitiate 
nor improve the atmosphere in which they are made 
to grow. 
307. But we have before seen, that, in the shade, 
plants, by their growth, uniformly convert the oxy- 
gen gas of the air into carbonic acid, and such con- 
version, we have argued, is the natural and necessary 
effect of vegetation. If, therefore, no acid be found 
in the air, when the process takes place in sunshine, 
either that gas cannot, under such circumstances, 
have been formed in vegetation, or, if it has been 
formed, it must at once have been reconverted into 
oxygen gas by the agency of the solar rays. Now 
since, in these experiments, the plants actually grew; 
since oxygen gas is necessary (30.) to vegetation, and 
* Recherches, p, 42. et 
