i 9 4 POPULAR SCIENCE MONTHLY. 



lialed. It will be of interest to note the relation of the living 

 world to the atmosphere. Eight hundred to nine hundred grammes 

 of carbon dioxide are produced in the respiration of a single 

 person for a day, and the entire product of the human race for 

 this period is twelve hundred million kilogrammes. In addition, 

 large quantities of the gas result from the combustion of the four 

 hundred and sixty millions of kilogrammes of coal and wood burned 

 yearly. The lower animals, fungi, and green plants themselves con- 

 tribute an amount which must bring the total to twice the immense 

 sum named above. The atmosphere contains three or four hun- 

 dredths of one per cent of carbon dioxide, or an amount of about 

 two to three thousand billions of kilogrammes. No especial varia- 

 tion in this proportion has been detected since observations upon this 

 point were first made. The fact that no increase takes place is partly 

 due to the absorption of the gas by plants, and its replacement by oxy- 

 gen, and also to certain geological processes in constant operation. 

 Absorption takes place at the rate of about two and a half grammes 

 for every square metre of leaf surface per hour, or about twenty-five 

 to thirty grammes daily, since the process goes on only in daylight. 

 It is to be seen that a single human being exhales as much carbon di- 

 oxide as may be removed from the air by thirty or forty square metres 

 of leaf surface. According to Ebermayer, a hectare (2.47 acres) of 

 forest would use eleven thousand kilogrammes of carbon dioxide 

 yearly, and the amount used by plants is generally much in excess 

 of that furnished by the activity of the inhabitants of any given area. 

 Plants thrive and show increasing vigor as the amount of carbon 

 dioxide in the air rises until two hundred times the present propor- 

 tion is reached. An increase of the gas in the atmosphere would 

 therefore be partly corrected by the absorption and by the stronger 

 vegetation induced. Nothing short of a comprehensive cataclysm 

 could work such disturbance to the composition of the air as to en- 

 danger the well-being of the animal inhabitants of the earth. 



The activity of a square metre of leaf surface results in the 

 formation of one and a half to two grammes of solid substance per 

 hour in sunlight. A vigorous sunflower with one hundred and forty- 

 five leaves constructed thirty-six grammes of solid matter in a day, 

 and a squash with one hundred and sixteen leaves one hundred and 

 sixteen grammes in the same length of time. The amounts formed 

 by such trees as the beech, maple, oak, poplar, elm, and horse-chest- 

 nut, with leaf surfaces aggregating three hundred to one thousand 

 square metres, must be correspondingly large. 



A comparison of plants grown in strong sunlight, diffuse light, 

 and darkness will reveal many differences in stature and internal 

 structure. These differences are for the most part due to the forma- 



