Page. — Atmospheric Carbon-dioxide and Leaf -development. 271 



and tends to show that it was burnt into CO 2 and found its way into the 

 atmosphere. 



Let us compare, then, the quantities of carbon at present existing in 

 the earth's crust and in the air respectively, in order to see how far the 

 aerial supply would be increased if all known carbon existed as CO 2 : — 



Percentage of CO 2 in air = by volume -04. 



Percentage of carbon in air = -04 x 3/11 = -Oil (nearly). 



Atmospheric carbon in poimds .qh 



per square inch of earth's x 15 = -0016. 



surface 



Percentage of C in earth's crust . . . . = -21 (F. W. Clark). 



Density of crust . . . . . . . . = say, 3. 



Terrestrial carbon in pounds per cubic inch of 62-5 x 3 -21 



crust 1728 ^ lOO " ■^^^^^• 



Atmospheric carbon per square inch of crust _ -0016 



Terrestrial carbon per cubic inch of crust "00022 ^" '' 



Hence, a column of the earth's crust 8 in. deep contains as much carbon as 

 a similar column of air of the full depth. 



Assuming the earth's crust to be fifty miles deep, if 1 per cent, of its 

 carbon was burnt and thrown into the air, the atmospheric CO 2 would be 

 increased more than three thousand five hundred times. 



The problem as to the amount of CO 2 formerly in the air may be con- 

 sidered also from another point of view. 



Plants are not and never have been the only absorbers of CO 2. The 

 weathering of rocks is ceaseless. Many igneous rocks consist largely of 

 siUcates of potassium, sodium, calcium, magnesium, and iron. CO 2 dis- 

 solved in rain-water converts these into carbonates. Hence the probability 

 of all the CO 2 found as carbonates in the sedimentary rocks being obtained 

 from the air. 



Hoobom estimates that the earth's limestones and dolomites contain 

 twenty-five thousand times as much CO.2 as the air. 



Chamberhn (American geologist), omitting the pre-Cambrian (pre-plant) 

 Hmestones, estimates that those formed since Ufe appeared contain twenty 

 to thirty thousand times as much CO 2 as the air. 



It is absolutely clear, therefore, that the air was once enormously richer 

 in CO 2 than now. Was this so when plants first appeared ? Carbon 

 separated from the air and now existing as fossil plants, coal, peat, &c., 

 contains hundreds of times as much carbon as is now in the air ; also, 

 the hmestones of fossihferous periods contain twenty to thirty thousand 

 times as much CO 2 as the air. 



Unless, therefore, there has been a regular supply of CO 2 from volcanic 

 vents or otherwise to balance this enormous removal of carbon, the atmo- 

 sphere at the period when plant-hfe began must have been excessively rich 

 in CO 2- Also, during the plant period the atmospheric CO 2 has been 

 reduced apparently in a steady continuous progressive manner from a very 

 large to its present very small proportion. 



One need not be a biologist or botanist in these days of popular scientific 

 literature and popular scientific lectures to be thoroughly famihar with the 

 fact that environment has a great deal to do with the form, character, and 

 habits of plants. Relatively small differences in temperature, in dryness 

 of air or of soil, in wind, in altitude, in presence of insects or herbivorous 



