as a Constituent of the Atmosphere. 391 



enormous amount of at least 50,000 millions of kilogrammes 

 of carbon dioxide. Dividing the absolute amount given above 

 by this number, we find that the amount of carbon dioxide in 

 the atmosphere would be double what it is at present in about 

 one hundred years if there were no means of compensation. In 

 arriving at this estimate no account has been taken of the 

 amount of oxygen used up in producing the dioxide. This 

 obviously affects the first three sources only; but by taking it 

 into account we should reduce the time somewhat; but practi- 

 cally this correction is so slight that it can be neglected. 

 Poggendorff made a similar calculation, and gave 386 years as 

 the period which it would take to double the amount of the 

 dioxide, supposing there were no compensating influences at 

 work. The discrepancy in the two numbers is explained, 

 first, by the absolute amount of C0 2 in the air being much 

 less according to my calculations than that previously sup- 

 posed, and also by the circumstance that Poggendorff 's esti- 

 mate of the amount yielded by the combustion of carbonaceous 

 substances was much less, owing to the defective data at his 

 command. 



Compensating Influences. 

 Having now arrived at an estimate of the amount of carbon 

 dioxide daily added to the atmosphere, let us examine the 

 causes which bring about its decomposition and removal from 

 the air. The known causes which are at work producing this 

 change may be considered under three heads, viz.: — 



(1) Fixation of carbon by growing plants. 



(2) Eemoval of dioxide by zoophytes. 



(3) Absorption of dioxide by inorganic chemical actions. 

 The first cause here mentioned is one which is essential to 



almost all forms of vegetable growth. In estimating its mag- 

 nitude we are met by the want of reliable experimental data, 

 making it almost impossible to arrive at any definite conclu- 

 sion. It is, however, the only one which restores the oxygen 

 to the atmosphere, in the other two actions the dioxide being 

 absorbed bodily without being decomposed. Also most, if 

 not all, the decomposition effected by plants will occur during 

 the spring and summer, the most active period of plant-growth. 

 The second and third causes act continuously. Certain expe- 

 riments have shown that a square metre of leaf will decompose 

 in sunlight about a litre of C0 2 . Also Mr. Trelawny Saun- 

 ders, some years ago, calculated for Sir Charles Lyell the area 

 of the land-surface of the globe. The figures he gives are*: — 



Total area of land 57,600,000 square miles. 



Area of Arctic and Antarctic land . 8,200,000 „ „ 

 * Ansted's 'Physical Geography,' p. xxxviii. 



