152 AIR AND LIFE. 



reactions effect the liberation of oxygen, water electrolysis, the decom- 

 position of chlorate of potassium, or of sulphuric acid under the influ- 

 ence of heat, for instance; but do any of these chemical processes, or 

 any others similar in result if not in method, occur in nature on any 

 important scale ? We do not know, but it seems doubtful. At all events, 

 since the composition of the atmosphere remains fairly constant, there 

 must be some agency by means of which the enormous mass of oxygen 

 which is daily, hourly, at every moment, absorbed in consequence of 

 the organic and inorganic combustions occurring over the whole globe, 

 is, sooner or later, returned to the atmosphere. Plants are the only 

 agency at present known by which this process is effected. At all 

 events they effect part of it. But are they equal to the task of effect- 

 ing the whole? The question has not been yet answered in quite 

 satisfactory terms. Mr. T. L. Phipson has recently endeavored to fill 

 this gap, and to show that plants are even a more important source 

 of oxygen than is commonly admitted. He cultivated a convolvulus 

 plant in an artificial atmosphere, entirely devoid of oxygen, but con- 

 taining some proportion of carbon dioxide, with the result that a part 

 of the latter gas disappeared, its place being taken by oxygen, which 

 can only have been evolved by the plant. Mr. G. Meyer had previously 

 expressed the opinion that oxygen is thus generated, but Mr. Phipson's 

 experiment is of great interest. The whole matter is very important, 

 for, if the oxygen contained in the atmosphere has been evolved by 

 plants, one may ask whether there has not been some time when the 

 atmosphere was very poor in oxygen and very rich in carbon dioxide, 

 and whether some time may not arrive when, conversely, the atmos- 

 phere will be well provided with oxygen and very deficient in carbon 

 dioxide. If such were to be the case, the equilibrium and homogeneity 

 of air, as far as its composition is concerned, would be very unstable 

 and temporary matters. But no answer of a satisfactory character can 

 yet be given to such questions. 



It may be added that, according to less recent data, 1 hectare (a 

 little over 2 acres) of forest exhausts each year the atmosphere of some 

 11,000 kilograms, or 5,596 cubic meters, of carbon dioxide, while in 

 return it yields nearly as much (5,594 cubic meters) oxygen. A field 

 of oats, similarly, returns about as much oxygen as it absorbs carbon 

 dioxide. Perhaps other agencies are at work and make up for the enor- 

 mous consumption of oxygen effected by human, animal, and plant res- 

 piration, and by inorganic combustions generally, and it does not seem 

 to us that adequate proof has yet been furnished that plants alone are 

 able to return to the atmosphere the oxygen which they, with all other 

 living beings, take from it. Leaving out of the question the subject of 

 the origin of oxygen, it is very difficult to ascertain the methods by 

 which, notwithstanding an enormous consumption, the ratio of this 

 gas remains fairly constant at the present time. 



While the proportion of oxygen in air is constant, or tolerably 



