CARBONIC ACID BY LIVING PLANTS. 
623 
The subjoined Table gives the results of a similar experiment with the leaves of Buxus 
sempervirens, which were found particularly suitable for the purpose from their activity 
and the hardiness with which they stand the hardships of their treatment without the 
least apparent injury. 
Leaves of Buxus sempervirens. 
I . 
Air . 
N . 
A . 
2 
7-11 
5-97 
1-14 
3 
4-74 
3-98 
0-76 
4 
4-38 
2*74 
1-64 
5 
2-79 
2-40 
0-39 
6 
1-86 
1-61 
0-25 
7 
2-32 
2-07 
0*23 
8 
1-53 
1-29 
0-24 
9 
1-22 
M 3 
0-09 
10 
1-09 
1-09 
0-00 
11 
0-95 
0-95 
0-00 
12 
0*87 
0-87 
0-00 
Similar but less-extended results were obtained with the leaves of Veronica salicifolia , 
which appeared to suffer from their treatment. I do not, therefore, quote them. 
The above Tables demonstrate the fact that exposure to air for one day markedly 
increases the amount of carbonic acid during many subsequent days. Hence the hypo- 
thesis (1) enunciated above expresses (partly at least) the true origin of the carbonic 
acid given off by plants when confined over mercury. It is really due in a great measure 
to prior oxidation. Some other general conclusions may be fairly educed from this 
result. 
It is of course well known that plants by exposure to air constantly absorb oxygen and 
exhale carbonic acid. Now the further fact may be added, that, of the oxygen absorbed 
during a day, part may not appear as carbonic acid till after the expiration of a week. 
This explains the circumstance noticed by De Saussure and others, that the volume of 
carbonic acid exhaled during a day is sometimes more and sometimes less than that of 
the oxygen absorbed. The actual process is not so simple as has been generally supposed. 
Thus, when a plant is observed during a certain day to absorb oxygen and give out car- 
bonic acid, the greater part of the latter is due to the oxygen absorbed during that day ; 
but some is derived from that absorbed the day previous, a smaller amount from the 
oxygen of the day previous to that, and so on back for several days. On the other hand, 
the oxygen absorbed is excreted as carbonic acid for the greater part on the same day, 
but by no means the whole ; for decreasing amounts will be the source of part of the 
carbonic acid exhaled by the plant during the ensuing week. 
But it is to be observed that the whole of the carbonic acid exhaled by the barks and 
leaves in the above experiments cannot be referred to previous oxidation. This is clearly 
shown by the foregoing tabular statements. Thus, in the case of the bark of Litsoeci 
oblonga, it is seen that the differences gradually diminish in value and finally disappear, 
while a considerable amount of gas is still daily exhaled. This gas cannot be attributed 
4 o 2 
