102 Observations on Licbig's " Organic Cliemistry^^ 



deep green of the leaves, and great vigour of the plants, must 

 have arisen from the absorption of carbonic acid by the roots, 

 without any help from humus or huniic acid. 



After producing the negative proofs which we have before stated, 

 of carbon not being derived from the earth by the roots, he pro- 

 ceeds next to state, as positive proofs of its being absorbed by 

 the leaves, that, though plants are continued to be grown on 

 soil, yet the quantity of carbon in the soil augments, in place of 

 diminishing. In forests, where the annual shedding of the 

 leaves causes an accumulation of organic matter, especially where 

 the trees are deciduous, not evergreen which do not shed their 

 leaves so frequently, this will be the case ; in the case of scourg- 

 ing crops carried off the land, it must certainly be the reverse, 

 very often at least. It would be difficult to persuade any nursery- 

 man who has taken a heavy scourging crop of old transplanted 

 ash from his ground, or a farmer who has scourged his ground 

 by successive crops of oats, that the ground contains more car- 

 bon than when the plants were first put on it. The particular 

 salts necessary to their growth being removed, as he says after- 

 wards, may be partly the cause, but carbon must be removed 

 also, and must be replaced as well as alkalies, though the action 

 of the air will restore carbon sooner than alkalies. 



He next states the quantity of oxygen gas consumed daily by 

 men and animals, and by the wood and coal employed as fuel ; and 

 notices the fact, that nevertheless the proportion of oxygen in the 

 air never varies, as corroborative of the fact that the leaves give 

 out oxygen. The oxygen consumed by animals and burning fuel 

 is returned to the atmosphere in the form of carbonic acid, which, 

 he thinks, must all be absorbed by the leaves of plants, the car- 

 bon fixed in the plant, and the oxygen returned ; and as part 

 only of the oxygen is returned, some of the oxygen being also 

 fixed in the plant, as proved by Dc Saussure, from the added 

 weight being more than the carbon consumed, the consumption 

 of carbonic acid by the leaves must be great. Carbonic acid gas, 

 however, is so much heavier than common air, its specific gravity 

 being r.5277, that it has a constant tendency to descend ; so 

 much so, that, in the caves which contain this gas naturally, a 

 dog will be killed, when a man, from his superior height, may 

 enter with impunity; and thus every shower that falls washes it 

 into the earth, and diminishes the quantity, and so would lead us 

 to infer that ])lants will get more by the roots than the leaves : 

 the upper strata of air around tall trees should not contain so 

 much, when it varies so much between the heiirhl of a doe; and 

 man. Lest it should be doubted that the quantity of carbonic 

 acid in the air would be sufficient to supply the leaves with 

 all the carbon, he estimates the quantity in the air from De Saus- 

 sure at yt/cjo'^'^ P^''^ '^^ '^•'' weight; and, if the whole were abs- 



