92 Mr. J. Stevenson on the Chemical and 



country air. Miintz and Aubin of Paris, who made or 

 collected a large number o£ the most reliable determinations 

 made in various parts of the world, found that the average 

 percentage of carbonic acid in the air of the Northern Hemi- 

 sphere (as calculated from the data supplied by the material 

 collected) was 0*0282 by volume, and that the average 

 percentage in the Southern Hemisphere was 0*0272. The 

 mean of these two percentages is 0*0277, which may therefore 

 be taken as the average for the atmosphere as a whole. This 

 figure, when reduced to percentage by weight, becomes 

 0*0423, carbonic acid gas being 1*528 times heavier than 

 ordinary air. At this percentage the total quantity of car- 

 bonic acid in the atmosphere (the weight of which, as we have 

 already seen, may be taken as 5200 x 10 1 - tons) becomes 

 2*2 x 10 12 tons, as stated above. 



Of the various agencies which affect the amount of car- 

 bonic acid in the atmosphere, either by adding to or re- 

 moving from it, probably the most important at the present 

 time are the growth of vegetation, on the one hand, and the 

 oxidation of vegetable and other organic remains, on the 

 other. So great must be the amount of carbonic acid 

 annually decomposed by vegetation, that it is doubtful if the 

 atmosphere contains enough carbonic acid to suffice (without 

 assistance) for 50 years' growth of vegetation at the present 

 rate. This can be readily shown by a simple calculation. 

 Let us take a very moderate estimate of the rate of growth of 

 vegetation on the land surface of the earth, viz. 1 ton of dry 

 wood or hay &c. per acre per annum, this being only one-half 

 of Liebig's estimate for ordinary meadow, forest, and agricul- 

 tural land, as stated in a previous article ; and let us leave the 

 vegetation that grows in the sea out of account altogether. 

 Now 1 ton of dry wood may be regarded as containing roughly 

 0'4 ton carbon, and to obtain 0*4 ton of carbon 1*466 tons of 

 carbonic acicl would have to be decomposed. The production 

 of 1 ton of dry wood &c. per acre per annum therefore 

 requires the decomposition of 1*466 tons carbonic acid per 

 acre per annum, which is equivalent to 939 tons per square 

 mile, or 46,950 x 10 6 tons for the whole land surface of the 

 earth, which may be taken in round numbers as 50,000,000 

 square miles. This figure (46,950 x 10 6 tons) is roughly the 

 47th part of the total weight of atmospheric carbonic acid as 

 given above. 



A very considerable quantity of carbonic acid must also 

 be removed from the atmosphere through the decompo- 

 sition of certain rocks by water and carbonic acid, a reaction 

 which has already been referred to. It is not very easv to 



