68 GENERAL BIOLOGY 



useful are bichloride of mercury, alcohol, and phenol 

 (carbolic acid). 



The Cycle of the Elements hi Organic Nature. 

 The ultimate source of the carbohydrates and fats 

 in plants and, hence, secondarily in animals, is, as 

 we have -seen, the carbon existing as CO 2 in the 

 atmosphere. There is reason to suppose that in 

 geologic time past, owing probably to great volcanic 

 activity then, the amount of CO 2 in the atmosphere 

 was much greater than it is at present. 1 At any rate 

 the proportion of CO 2 in the air nowadays is sur- 

 prisingly small, not more than .05 per cent, of 

 which the carbon itself, the part utilized by the 

 plant, constitutes but a little more than one fifth. 

 The botanists have calculated that the cellulose in 

 a single dried tree trunk weighing 11,000 Ibs., repre- 

 sents a carbon moiety of 5500 Ibs., and that to secure 

 this amount of carbon such a tree must have drained 

 more than 16,125,000 cubic yards of air of its CO 2 . 

 Meteorologists calculate that, although the atmos- 

 pheric envelope of the earth may extend (in a very 

 tenuous state) several hundred miles above the sur- 

 face, yet -seven eighths of it by weight lies under a 

 height of 10.2 miles from the ground. Allowing 

 for the diffusion of its constituents, we may estimate, 

 for the sake of the argument, that the surface vege- 

 tation can draw tribute of CO 2 from a height of ten 



1 Geologists have even ascribed the initiation of the glacial epochs 

 to the decrease of COj in the atmosphere, consequent in part upon the 

 great development of plant life in preceding epochs. Sec Chamberlain 

 and Salisbury, Geology, III, p. 424. 



