100 Mr. J. Stevenson on the Chemical and 



great amount of work in connexion with the question of 

 local variations in the percentage of carbonic acid and 

 allied subjects, once observed (as quoted by Prof. Letts 

 in his work on the Carbonic Anhydride of the Atmosphere) 

 that the u crown air " of a tall pine-tree in a plantation 

 in the Karsten forest showed a very low percentage of car- 

 bonic acid, so low that he was not willing to accept the 

 result ; and he was of opinion that some of the carbonic 

 acid had combined with the lead of the long tube used in the 

 experiment, the sides of the tube having become moist. 

 Whether this experiment should have any positive value or 

 not, it is quite credible that on a very calm bright day the 

 percentage of carbonic acid in the crown-air of a tree in full 

 foliage may become much reduced by the process of respi- 

 ration, and therefore the growth of the tree may be less 

 rapid than it would be if the carbonic acicl were kept up to 

 or near its normal percentage by the circulation of the air, 

 or otherwise. At the same time, the limits imposed on the 

 luxuriance of vegetation in those parts of the world where it, 

 is poor seem not to have much direct reference to the amount 

 of carbonic acid in the atmosphere, but rather to the con- 

 ditions regarding temperature, sunlight, rainfall or water- 

 supply of some kind, and fertility of the soil. It is quite 

 possible, however, that in regions where vegetation is very 

 luxuriant, a certain amount of restriction is imposed by the 

 limited quantity of atmospheric carbonic acid, especially if 

 the weather should frequently be very calm. In the forests 

 of the Amazons and other luxuriant forests, where the 

 struggle for life on the part of vegatation is to a great extent 

 a struggle for light, the amount of growth is largely con- 

 ditioned by the amount of foliage that can reach the sunlight ; 

 and if the percentage of carbonic acid in the air at the tops 

 of the trees should be much reduced in calm weather by the 

 respiration of the leaves, it is very probable that the growth 

 of the trees will be thereby to some extent affected. 



However, it is highly probable that considerations of this 

 kind do not after all affect the main question before us very 

 seriously; for we have to do not so much with the total 

 amount of vegetable growth per annum or the total amount 

 of carbonic acid annually decomposed and otherwise removed 

 from the atmosphere, but rather with the difference between 

 the amount annually removed by vegetation and otherwise 

 and the amount annually restored by eremacausis and other 

 modes of oxidation. Now it is quite reasonable to suppose 

 that if the luxuriance of vegetation were to be very greatly 

 increased, the amount of carbonic acid arising from the 



