1 1 8 Observatio7is on Liehig^s " Organic Chemistry y 



plants, they collect alkalies, he says, as well as those of the land. 

 Plants inland collect the alkalies evaporated from the sea in 

 rain water, and from springs that permeate the earth. Were it 

 not for plants, he concludes, alkalies would gradually disappear, 

 though he does not state how; perhaps he means by again 

 forming the earths into stones, the metals do not part with their 

 oxygen readily. 



On the next division, the Art of Culture, he resumes the con- 

 sideration of the use of humus; its insolubility, he says, pre- 

 vents its being carried off from ground, being soluble only when 

 combined with oxygen, and being taken up by water only as 

 carbonic acid. The humus in the soil unites to oxygen, and 

 gives off carbonic acid ; which, as before said, stops the further 

 decay of the humus, till the carbonic acid formed is taken off 

 by plants; when a fresh supply of oxygen resumes the action on 

 the humus. He next quotes various places which abound in ve- 

 getable remains, and calcareous earth ; and, from the fact of no 

 humic acid, or humate of lime, being found in these places, infers 

 its absence in common vegetable mould ; it is carbonic acid, not 

 humic, he says, that gives the food to plants. I think it is 

 most likely to be the way, at least for the greatest part. Dr. 

 Madden, on the same subject, says that, though a solution of 

 soil in water will not yield humic acid, yet it will yield it to a 

 solution of salts ; and infers that it exists in small quantities, and 

 forms gradually humate of lime : the salts, however, in this ex- 

 periment, may have acted on the humus, and produced part at 

 least of the humic acid detected. The humus, Dr. Liebig says, if 

 in great quantity, will rob the ground of its oxygen ; and he points 

 out some places where parts of meadows are burnt up, and 

 stinted in vegetation, from the great quantity of carbonic acid ; 

 so great at times as to be emitted with an explosion, when the 

 ground is bored into. Part of the cause of this ought perhaps 

 also to be ascribed to excess of food, which will kill many plants ; 

 but he inclines to refer it solely to the deprivation of oxygen ; and 

 illustrates this position by the effects of stagnant water, which 

 having parted with its free oxygen, and being stagnant, and the 

 oxygen consequently not renewed from the air, plants languish 

 and die for want of oxvijen when their roots are confined in it. 

 This, I believe, is fully borne out in practice. It has been cus- 

 tomary to say that alders, willows, and other plants, will thrive 

 in stagnant water; but I am informed by Mr. Aitken of Lanfine, 

 in this neighbourhood, Newmilns, that the plantations there, 

 which are to a vast extent on moorland, and much of it very 

 marshy, completely refute the common opinion, and show the 

 necessity of oxygen as well as moisture. The alders, willows, 

 poplars, ash, &c., though they require moisture to their roots, 

 will not thrive where it is stagnant; and it is only near the little 



