118 ELEMENTS OF BOTANY. 
-148. Absorption of Carbonic Acid Gas. — Carbonic acid gas 
is a constant ingredient of the atmosphere, usually occurring 
in the proportion of about 4 parts in every 10,000 of air or 
ss of one per cent. It is a colorless gas, a compound of two 
simple substances or elements, carbon and oxygen, the former 
familiar to us in the forms of charcoal and graphite, the latter 
occurring as the active constituent of air. 
Carbonic acid gas is produced in immense quantities by the 
decay of vegetable and animal matter, by the respiration of 
animals, and by all fires in which wood, coal, gas, or petro- 
leum is burned. 
Green leaves and the green parts of plants have the power 
of removing carbonic acid gas from the air (or in the case of 
some aquatic plants from water in which it is dissolved) and 
setting free part or all of the oxygen. ‘This process is an 
important part of the work done by the plant in making over 
raw materials into food from which it forms its own substance. 
‘149. Experiment 28. Oxygen-Making in Sunlight. — Place a 
green aquatic plant in a glass jar full of fresh water, in front of a sunny 
window.! Note the rise of oxygen bubbles. Remove to a dark gloset 
for a few minutes and examine by lamplight, to see whether the rise of 
bubbles still continues. 
- This gas may be shown to be oxygen by collecting some of 
it in a small inverted test-tube filled with water, and thrust- 
ing the glowing coal of a match just blown out into the gas. 
It is not, however, very easy to do this satisfactorily before 
the class. 
Repeat the experiment, using water which has been well boiled and 
then quickly cooled. Boiling removes all the dissolved gases from water, 
and they are not re-dissolved in any considerable quantity for many 
hours. 
Ordinary air, containing a known per cent of carbonic acid gas, if 
=a Elodea, Myriophyllum, Chrysosplenium, Fontinalis, any of the aquatic green 
flowering plants, or even the common confervaceous plants known as pondscum or 
“frog-spit,” will do for this experiment. 
