CARBONIC ACID BY LIVING PLANTS. 
621 
being secured in one neck by means of a perforated stopper of soft caoutchouc with a 
side slit for the introduction of the stem, so that, being firmly thrust into the neck, it 
Pig. 4. 
made an air-tight joint. The issuing gas then slowly bubbled through a potash bulb, 
which was periodically weighed — or else, when a test was required, through baryta-water. 
Contamination with the external air was prevented by finally allowing the gas to go 
into distilled water. 
The gas was passed before regular determinations were made. During this period 
occasional tests were made with baryta-water, which always showed the presence of car- 
bonic acid. On the seventh day the gas was passed through the potash bulbs, which 
were weighed night and morning. The following were the results : — 
By day, 
weight in grammes. 
By night, 
weight in grammes. 
7th day 
0-0186 
0-0202 
8th „ 
0-0362 
0-0532 
9th „ 
0-0375 
0-0335 
10th „ 
0-0395 
0-0133 
11th „ 
0-0372 
0-0152 
12th „ 
0-0183 
0-0125 
13th „ 
0-0566 
0-0147 
14th „ 
0-0089 
0-0194 
1 6th „ 
0-0464 
0-0194 
17th „ ; 
0-0186 
0-0016 
18th „ 
0-0210 
0-0121 
The foregoing numbers indicate that the branch of Erica evolved carbonic acid long 
after being excluded from the influence of oxygen. The mean amount during the last 
eleven days was somewhat over 1 cub. centim. per hour. In this experiment the exhaled 
carbonic acid, being mingled with a large amount of inert nitrogen, was liable to be 
decomposed by the plant during day. This circumstance was doubtless a source of loss 
of carbonic acid, but it was unavoidable. 
The excretion of carbonic acid by plants has generally been ascribed to a process 
analogous to the breathing of animals, oxygen being absorbed and carbonic acid exhaled. 
The greater part of the carbonic acid exhaled has doubtless this origin, but it is clear 
from the foregoing experiments that it will not account for the whole. They show that 
mdccclxix. 4 o 
