394 Carbon Dioxide as a Constituent of the Atmosphere. 



Thus, Dr. Hunt says " that a weight of carbonic dioxide equal 

 to more than twenty-one times that of our present atmosphere 

 would be absorbed in the production from orthoclase of a layer 

 of kaolin extending over the earth's surface with a thickness 

 of 500 metres, an amount which evidently represents but a 

 small proportion of the results of felspathic decay in the sedi- 

 mentarv strata of the globe/' Evidently, then, here we have 

 a cause which has removed, and is removing, a vast amount of 

 carbon dioxide from the atmosphere. Any estimate of the 

 rate of its action is obviously impossible. It must not be for- 

 gotten, however, that subaerial felspathic decay is a very slow 

 process, and that therefore the large deposits of decomposed 

 felspar found in the earth seem to point rather to a compara- 

 tively slow process acting through an immense number of years 

 than to a rapid process such as that effected by plants. 



General Conclusions. 



It, is of course evident that, if the compensating influences 

 are just equal in amount and in rate of action to the producing 

 ones, the amount of carbon dioxide in the air will remain con- 

 stant. Unfortunately an insufficiency of reliable data prevents 

 a definite answer being given to such a question. The fore- 

 going considerations, however, seem to show that in all pro- 

 bability the causes at work removing atmospheric dioxide are 

 more powerful than those producing it. As a consequence, 

 the atmosphere is being robbed of this constituent, the greater 

 part of which is becoming fixed in the solid -earth as carbonate 

 of lime. But this process has already gone on for so long a 

 time, that there is already fixed in this way an immense quan- 

 tity of C0 2 equal to many hundreds of times the amount con- 

 tained in the existing atmosphere. The question of the source 

 of this large amount naturally arises; but the answer to be 

 given must simply be.an admission of our want of knowledge. 

 The idea that it all at one time formed part of the atmosphere 

 of the globe has been suggested by Brongniart; and Dr. 

 Sterry Hunt considers (loc. cit.) that a universal atmosphere 

 of the same quality as that of the earth exists, from which the 

 carbon dioxide now fixed in the earth's crust has been derived. 



There can be no doubt that, unless we accept the latter of 

 these theories, there must at some antecedent period have been 

 an atmosphere covering the globe much richer in this gas than 

 the present one ; but whether such an atmosphere would 

 account for the luxuriant vegetation of the Coal Period is at 

 present an open question. If Dr. Hunt's hypothesis be a cor- 

 rect one, it is interesting to remember that the carbon which 

 we contain in our bodies may have existed at one time as a 



