235 
little, if at all perceptible. He placed three plants 
of tysimachia vulgar is, weighing 129^- grains, in a re^ 
cipient containing 250 cubic inches of common air 
deprived of its carbonic acid, the roots of the plants 
being plunged in a little distilled water. The plants 
were then kept, alternately in sunshine and in the 
shade, for eight days, at the end of which time they 
were perfectly sound, and had grown considera- 
bly, but had not at all affected the purity or the 
volume of their atmosphere. In their green state, 
they now weighed 141 grains, and, after being dried 
in the common temperature of the atmosphere, their 
weight was only 401 grains. Similar plants, precise- 
ly of the same weight, were, at the same time, gather- 
ed and dried in the same manner, and were found to 
weigh 38 grains ; so that those plants, which had 
grown eight days in confined air, exceeded them in 
weight, when both were reduced to the same state 
of dryness, only by two grains. These comparative 
experiments he repeated many times on different 
plants, and prolonged them to a fortnight or even a 
month ; but he never found the weight of the con- 
fined plants to exceed that of others, which had not 
been confined, by more than two grains, and some- 
times not at all, although they had, in some cases, 
grown several inches in length *. 
519. To these experiments it may, perhaps, be ob- 
jected, that the difficulty of reducing plants to a si- 
milar degree of dryness prevents us from arriving at 
any very accurate knowledge of their solid substance, 
* Recherches, p. 221. 
