82 



FOREST INFLUENCES. 



tliis case) one million times that depth or volume of air, the latter being 

 of average temperature and density. 



In calculating the effect of the breaking up of the carbonic acid, we 

 may assume that as much heat is used as would be given out by burn- 

 ing the same quantity of carbon in oxygen— that is, by remaking the 

 compound. Grandeau gives as the annual product of carbon by 

 plants, per hectare, 3,000 kilograms for forests, 1,500 to 4,500 kilograms 

 for open fields, and, exceptionally, 15,000 kilograms for a field of giant 

 maize. This is probably all derived from the carbon dioxide of the air. 

 The burning of a kilogram of carbon produces 7,900 large calories of 

 heat according to Andrews, or 8,080 according to Favre and Silber- 

 mann. 



The large calorie is the amount of heat necesssry to raise one kilo- 

 gram of Avater 1^ C. in temperature; it would warm to the same amount 

 1 -^ 0.237 = 4.22 kilograms, or 4.22 x 773.3 = 32G3.3 liters of standard 

 air. The burning of the annual carbon productions above given would 

 warm by 1° C. the thickness of air mentioned in the following table, in 

 Avhich 7,900 is used as the number of calories produced by the combus- 

 tion of 1 kilogram of carbon : 



Forest. 

 Prairie 

 Maize . 



Ciilorics 



used per au- 



iiuiii ])cr 



hectare. 



23,7 X 10 « 



<, 11. K X 106 



i 35. 5 X 10« 



118.5 X 106 



Liters of air 

 cooled. 



77.2 X IQS 



38. 6 X 10' 



115.8 X 109 



380.0 X 109 



Thickness 

 of air 

 cooled. 



Miles. 



5.0 



2.5 



7.5 



25.0 



It appears that tlie heat absorbed by the entire carbon assimilation 

 for a forest is only from one-fifth to one-sixth of that used in evaporat- 

 ing a layer of water an inch deep. As the latter is only a sixth or a 

 seventh of the transpiration, it appears that the cooling which would 

 be caused by the breaking up of carbonic acid, in order to form forest 

 growth, is only from one-thirtieth to one-fortieth of that caused by the 

 transpiration. 



It is highly probable that the heat used in the former process is more 

 or less closely made up by the heat produced in the oxidations that go 

 on in the plant. In any case the cooling due to this source may be 

 disregarded and only that of the transpiration be considered. It is es- 

 timated that the sun's rays, when the sun is vertical, pour out twenty- 

 five calories per minute over each square meter of surface exposed ver- 

 tically to it. Of this about 30 per cent is absorbed in passing through 

 the atmosphere, when the sun is vertical and the sky clear, and very 

 much more under other circumstances. The 17.5 calories, for a dear 

 day and vertical sun, would in a single minute evaporate a layer of 

 water 0.0011 inch thick. But the transpiration is about 0.5 inches 

 for the season of about 150 days, or 0.00003 per minute. Thus the 



