118 ELEMENTS OP 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 

 ■^ 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 SunligM. — Place a 

 green aquatic plant in a glass jar full of fresh water, in front of a sunny 

 window. 1 Note the rise of oxygen bubbles. Remove to a dark closet 

 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, it 



* Elodea^ Myriophylhmi, Chrysospleninm, Foiitinalh. any of tlie aquatic green 

 flowering plants, or even t5ie common confervaceous plants known as pondscum or 

 "frog-spit," will do for this experiment. 



