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I may, perhaps, appropriately in this connection refer 

 to a question suggested by this experiment, viz. How is 

 the supply of carbonic acid gas in the atmosphere main- 

 tained ? The tendency of terrestrial vegetation, especially 

 on forest areas, is not merely to absorb immense quantities 

 of carbonic acid gas for structural purposes, but, as we 

 see so largely evidenced in our coal formations, to lock up 

 a considerable proportion of the solid carbon, and put it, 

 as it were, out of circulation. The popular assumption 

 is that equilibrium is maintained by the fact that the 

 animal kingdom exhales carbonic acid gas and the vege- 

 table kingdom absorbs it and exhales oxygen, but although 

 this may account for a partial restoration, it is to my mind 

 a very partial one indeed, since there is not only the 

 annual lock-up of carbon aforesaid, but when we consider 

 the vast amount of the earth's surface occupied by vegeta- 

 tion and the comparative paucity of animal life, the idea 

 of any adequate compensation falls away. Vast areas of 

 forest land are all but solitudes, while the foliage must 

 absorb every day during the growing season an enormous 

 amount of carbonic acid gas, to judge by the growth of the 

 timber. To get at some idea of this, I applied to the well- 

 known authority on Forestry, Mr. A. D. Webster, who 

 informed me that the annual accretion of timber on a 

 square mile of forest land amounted to from 10,000 to 

 13,000 tons, according to the tree species, on the very 

 moderate estimate of one cubic foot per tree. On the other 

 side I turned to Prof. Huxley's estimate of the amount of 

 solid carbon as a constituent of the carbonic acid in the air 

 over such an area, and found it to be about 3,700 tons 

 carbon (Physiography, p. 84). Huxley estimates that wood 

 contains about half its weight of carbon, which means 

 that in much less than a year a forest area would entirely 

 exhaust the local supply, and following this up, we have only 

 to divide the entire earth's surface by the torestal area of 



