ATMOSPHERIC AIR AS THE FOOD OF PLANTS. 4] 
The Absorption of Carbonic Acid by Piants.—In 1771 
Priestley, in England, found that the leaves of plants im- 
mersed in water, sometimes disengaged carbonic acid, 
sometimes oxygen, and sometimes no gas at all. <A few 
years later Ingenhouss proved that the exhalation of car- 
bonic acid takes place in the absence, and that of oxygen 
in the presence, of solar light. Several years more elapsed 
before Sennebier first demonstrated that the oxygen which 
is exhaled by foliage in the sunlight comes from the car- 
bonic acid contained in the water in which the plants are 
immersed for the purpose of these experiments. It had 
been already noticed, by Ingenhouss, that in spring water 
plants evolve more oxygen than in river water. We now 
know that the former contains more carbonic acid than the 
latter. Where the water is by accident or purposely free 
from carbonic acid, no gas is evolved by foliage in the 
sunlight. ; 
The attention of scientific men was greatly attracted 
by these interesting discoveries; and shortly Percival, in 
England, found that a plant of mint whose roots were 
stationed in water, flourished better when the air bathing 
its foliage was artificially enriched in carbonic acid than in 
the ordinary atmosphere. 
In 1840 Boussingault furnished direct proof, of what 
indeed was hardly to be doubted, viz.: the absorption of 
the carbonic acid of the atmosphere by foliage. 
Into one of the orifices in a three-necked glass globe he introduced 
and fixed air-tight the branch of a living vine bearing twenty leaves; 
with another opening le connected a tube through which a slow current 
of air, containing,in one experiment, four-10000ths of earbonie acid, 
could be passed into the globe. This air after streaming over the vine 
leaves, at the rate of about 15 gallons per hour, escaped by the third 
neck into an arrangement for collecting and weighing the carbonic acid 
that remained in it. The experiment being set in precess in the sun- 
light, it was found that the enclosed foliage removed from the current 
of air three-fourths of the carbonic acid it at first contained. 
Influence of the Relative Quantity of Carbonic Acid,— 
De Saussure investigated the influence of various propor- 
