42 HOW CROPS FEED. 
tions of carbonic acid mixed with atmospheric air on the 
development of vegetation. He found that young peas (4 
inches high) when exposed to direct sunlight, endured for 
some days an atmosphere consisting to one-half of carbonic — 
acid. When the proportion of this gas was increased to 
two-thirds or more, they speedily withered. In air con- 
taining one-twelfth of carbonic acid the peas flourished 
much better than in ordinary atmospheric air, The aver- 
age increase of each of the plants exposed to the latter 
for five or six hours daily during ten days was eight 
grains ; while in the former it amounted in the same time to 
eleven grains. In the shade, however, Saussure found that 
increase of the proportion of carbonic acid to one-twelfth 
was detrimental to the plants. Their growth under these 
circumstances was but three-fifths of that experienced by 
similar plants exposed to the same light for the same time, 
but in common air. He also proved that foliage cannot long 
exist in the total absence of carbonic acid, when exposed 
to direct sunlight. 'This result was obtained by enclosing 
young plants whose roots were immersed in water, or the 
branches of trees stationed in the soil, in a vessel which 
contained moistened quicklime. This substance rapidly 
absorbs and fixes carbonic acid, forming carbonate of lime. 
Thus situated, the leaves began in a few days to turn yel- 
low, and in two to three weeks they dropped off. 
In darkness the presence of lime not only did not de- 
stroy the plants, but they prospered the better for its 
presence, 1. e., for the absence or constant removal of car- 
bonie acid. 
Boussingault has lately shown that pure carbonic acid 
is decomposed by leaves exposed to sunlight with extreme 
slowness, or not at all. It must be mixed with some other 
gas, and when diluted with either oxygen, nitrogen, or hy- 
drogen, or even when rarefied by the air-pump to a certain 
extent, the absorption and decomposition proceed as usual. 
Conclusion, —It thus is proved Ist, that vegetation 
