43 
259. But M. Senebier goes still farther, and main- 
tains, that the carbonic acid, which is thus formed 
by plants, when they are confined in closed vessels, 
does not proceed from the plant. It is, says he, 
more abundantly produced in the shade than in the 
light, and always, in both cases, when the heat is 
greatest. The atmosphere of such plants diminishes, 
and loses precisely the quantity of oxygen gas neces- 
sary to form the carbonic acid which we find ; so 
that this acid is necessarily composed of the oxygen 
which is exterior to the plant, and of carbon that has 
escaped from the vegetable to unite with it. If, he 
adds, the carbonic acid had proceeded directly from 
the plant, the atmosphere would have been vitiated 
without being changed, and the proportion of oxy- 
gen gas would still have been found in it, after its 
acid gas had been removed by washing *. 
260. In the year 1797, M. T. de Saussure pu- 
blished some experiments on the vegetation of plants 
in atmospheric air. He raised some garden peas (pi- 
sum sativum) in water, till they attained the height of 
between three and four inches. These plants were 
then put into a glass containing water, in which their 
roots were immersed. This glass was next placed 
on a saucer filled with water, and over it a recipient, 
containing 50 cubic inches of atmospheric air, was 
inverted. The apparatus was then set aside in a 
room, but did not receive the direct rays of the sun. 
At the end of ten days, the plants had considerably 
increased in weight ; the volume of air, in which 
* Physiol, Ycg. vol. iii, p. 121, 
