V. The Iufluence of Light upon the Growth of Leaves. 
12» 
Into the one saucer, d\ a strong solulion of potash is poured, and into 
the olher, water. Each of the plants thus arranged is covered with a 
bell-jar (c', c",) which is firmly fixed in its position by one of the upper 
pair of rings which slide on the rod. Through the cork of the bell-jar 
c" passes a tube which allows free communicalion between the externa! 
air and the interior of the bell-jar. Two tubes pass through the cork of 
the other bell-jar, c', 
each of which is con¬ 
nected by rneans of an 
india-rubber-tube, with 
a wash-bottle containing 
lime-water (6', 6"). The 
wash-bottle b" commu- 
nicates with the air by 
the shorter of the two 
tubes which enter it, 
but the wash-bottle b" 
is in connection with a 
large tube A filled with 
purnice - stone soaked C^I 
with concenlrated So¬ 
lution of potash, and the tube A is connected with an aspirator which 
is not shewn in the drawing. The aspirator is so arranged that, when 
it is in action, air is not drawn through the apparalus into the aspirator, 
but it is forced from the aspirator into the apparatus. By this means 
any possible entrance of air at the weak point of the apparatus, that is, 
at the place where the stein of the plant is fixed in the opening of the 
saucer, is effeclually prevented, for, when air is being passed through 
it, the pressure within the apparatus is slightly greater than that of the 
atmosphere. 
ln the course of the experimenls the aspirator was not kept con- 
stantly in action, but it was uscd regularly every morning before the 
plant was exposed to light, in order to remove all the carbonic acid 
which had been evolved during the night and which had remained un- 
absorbed, and it also allowed to run two or three times during the day. 
The air forced into the apparatus frorn the aspirator had to pass over 
the puinice-stone soaked with solulion of potash contained in the tube 
A, and, in order to test its freedom from carbonic acid, it bad to bub- 
ble up through the lime-water contained in the wash-bottle b'. In all the 
experiments thus performed I never perceived the least precipilale in this 
wash-bottle, so that the air which reached the bell-jar c' could not have 
contained any carbonic acid. The air which was forced out of the bell- 
jar c' was conveyed to the surface of the lime-water in the w'ash-bollle 
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