ATMOSPHERE IN RELATION TO HUMAN LIFE AND HEALTH. 307 



open to attack in different seasons, according to the weather; (3) the 

 treatment of the ground in drought with a view to destroy threatening 

 pests in their early stages, and, in general, the conduct of agricultural 

 operations with regard to the probable development of particular pests 

 resulting from particular kinds of weather; (4) the issue of forecasts 

 of insect prevalence, derived from a careful study of the habits of 

 various species of insect pests, and of the weather of present ami 

 previous seasons. 



ACTION OF PLANTS ON THE AIR. 



Plants in general take up free oxygen from the air and during the 

 night exhale a small quantity of carbon dioxide. They also give a 

 large quantity of oxygen to the air by the breaking up of carbon 

 dioxide into carbon and oxygen through chlorophyll. The oxygen is 

 set free, while the carbon is retained. Experiments have been made on 

 various plants with the object of ascertaining the amount of oxygen 

 which they absorbed at different temperatures. The following are 

 some of the results : 



Five seedlings of Tropceolum majus absorbed 1.04 cubic centimeters 

 carbon dioxide of oxygen per hour at 35° 0. 



Four seedlings of wheat absorbed 0.088 cubic centimeter of oxygen 

 per hour at 15.4° C. 



Each plant has its temperature of maximum absorption. Wheat 

 evolved 37.6 milligrams of carbon dioxide per hour at 40° O. The maxi- 

 mum amount of carbon dioxide evolved at the temperatures does not 

 correspond with the maximum of oxygen absorbed. Variations in the 

 composition of the atmosphere do not interfere with the respiration of 

 plants, and the relations of the amounts of these gases absorbed and 

 evolved, unless those variations are extreme, and not occurring in 

 natural conditions. 



Plants have been placed under glass shades, with tbeir roots immersed 

 in water containing free carbonic acid and certain salts, and with their 

 upper parts exposed to a north light in carbon dioxide, hydrogen, and 

 nitrogen. In the carbon dioxide they did not thrive. Convolvulus 

 throve very well in nitrogen, mixed with a third part of carbon diox- 

 ide, and after three weeks these gases were found to be mixed with so 

 much oxygen as to approach the proportions in the atmosphere. The 

 power of plants to produce in a closed space an atmosphere resembling 

 that of the globe might well form the subject of research on a great 

 scale. 



THE INFLUENCE OF FORESTS ON CLIMATES. 



The influence of forests on climate is now much better ascertained 

 than it was thirty years ago, at any rate with regard to temperate 

 regions. But the importance of preserving trees, woods, and forests 

 is far from being recognized as it ought to be by Governments and by 

 the people generally: 



