466 Comparative Physiology. 



acid from the air : the air above water always contains less car- 

 bonic acid from this cause. On the whole, therefore, it appears 

 that though undoubtedly carbonic acid is absorbed by the leaves 

 of plants, it seems wrong to say that it is wholly got in that 

 way, and doubtful whether it is the principal source, much of 

 what is got from the air being probably that washed down by 

 the rain and absorbed by the roots. Some plants are no 

 doubt able to prolong their existence without having their roots 

 fixed in the soil ; but the growth is not vigorous, and no prac- 

 tical man will be of opinion that it is of no use to bury the 

 carbon of the manure in the soil. When a tree has its stem cut 

 by the ground, if the roots are left untouched, the after-growth 

 is so vigorous as soon to fill the former space, and even produce 

 more wood than if it had been left uncut ; but if the roots are 

 cut off it languishes and dies, though supplied with moisture. 

 Cuttings never grow much till they begin to root, and those 

 that do so first in a pot of cuttings always outstrip and kill the 

 others. In houses, from the air being confined, and not so 

 frequently renewed, the cai'bonic acid of the air cannot be 

 furnished in so great quantity ; yet, if vines are well supplied 

 at the root, they are not found to suffer from a want of carbon 

 in the air of the house. Liebig, in his Agricultural Chemistry, 

 says most of the carbon from the air to the leaves is pro- 

 bably from carbonic oxide ; and this, from its being much lighter 

 than carbonic acid, is probably the case. Carbonic oxide, as 

 compared with atmospheric air, is only 0*972, carbonic acid 1*527, 

 reckoning air as 1. Another source of carbon to plants is 

 humic acid, which contains much more carbon than carbonic 

 acid. From its extreme volatility and aptitude to change, and 

 from its being seldom got in analysis of soils, Liebig and other 

 chemists have stated their opinion that it cannot be a great source 

 of carbon to plants, and that the humus of the soil is principally 

 converted into carbonic acid. Professor Sprengel, however, 

 who, with great scientific knowledge, appears to have studied the 

 subject much in practice, seems to consider it a great source of 

 food ; and recommends the alkalies in composts, principally on 

 account of their tendency to cause the formation of humic acid 

 from the humus of the manure, in place of carbonic acid. In 

 the excellent papers copied from his works into the Gardener's 

 Chronicle, humic acid appears to be considered a most essential 

 part of the composts he describes. The experiments of Dr. 

 Madden, also, have thrown great light on this subject. He found 

 that in passing ammonia through humus or soil containing 

 humus, humic or ulmic acid was always formed in quantity 

 corresponding to the ammonia introduced, and was found in the 

 water drained ofi" in the state of ulmate of ammonia. The 

 action of ammonia on humus (or semidecomposed vegetable 



