CRENIC AND APOCRENIC ACIDS. 279 



has very properly objected to this opinion,* that they are so very sparingly 

 soluble in water that we cannot suppose them to enter directly into the 

 roots — even were all the water they absorb to be saturated with them — 

 in such quantity as to contribute in a great degree to the organic matter 

 contained in almost any crop.f 



We have indeed seen reason to conclude on other grounds, that only a 

 small, though a variable, proportion of the carbon of plants is derived 

 from the soil, yet of this proportion a certain quantity may enter by the 

 roots in the form of one or other of these acids, or of their earthy com- 

 pounds. They are readily soluble in ammonia ; and animal manures 

 which give off this compound in the soil may therefore facilitate their 

 entrance into the roots of those plants which are cultivated by the aid of 

 such manures. They are also soluble in carbonate of potash and car- 

 bonate of soda, which are contained in wood ashes and in the ash of 

 weeds and of soils which are pared and burned. When these substan- 

 ces, therefore, are applied to the land, they may combine with, and, 

 among their other beneficial modes of action, may serve to introduce, 

 these acids in larger quantity into the plant. 



When exposed to the air, the humates and ulmates contained in the 

 soil undergo decomposition, give off carbonic acid, and are changed into 

 carbonates. The admission of air into the soil facilitates this decompo- 

 sition, which is supposed to be continually going forward — and it is in the 

 form of this gas that plants are considered by some to imbibe the largest 

 portion of that carbon for which they are indebted to the soil. 



4°. Crenic and Aprocrenic acids. — When soils are digested or washed 

 with hot water, a quantity of organic matter is not unfrequently dissolved, 

 which imparts to the water a brownish yellow colour. When the solu- 

 tion is evaporated to dryness, there remains besides the soluble saline 

 substances of the soil, a variable portion of brown extractive looking 

 matter also, which is a mixture of the two acids here named, with the 

 ulmic and humic — all in combination with lime, alumina, and other bases. 

 When this residue is dried at 230° F., the two latter acids, and their 

 compounds, become insoluble, while the crenates and apocrenates, more 

 esjjecially the former, remain soluble in water, and may be separated 

 by washing with this liquid. 



These acids also are formed in the soil during the decay of vegetable 

 matter. They are distinguished from the two previously described by 

 containing nitrogen as an essential constituent, and by forming compounds 

 with lime, &:c., which are, for the most part, readily soluble in water. 

 Hence th^ will both prove more nourishing to plants — in virtue of the 

 nitrogen they contain — and in consequence of their solubility, will be able, 

 where they exist, to enter more readily, and in greater abundance, into 

 the roots than either the ulmic or the humic acid. 



Owing to this solubility, also, they are more readily washed out of the 

 soil by the rains, and hence are rarely present in any considerable quan- 



• Organic Chemistry applied lo Agriculture, first edition, pp. 11 and 12. 



t Ulmic acid requires 2500 times its weiglit of water to dissolve it— ulmate of lime 2000 

 times, and ulmate of alumina 4200 times — but all are still less soluble after they have been 

 perfectly dried, or exposed to the action of a hard winter's frost. The ulmates of potash, 

 ■oda, and alumina, are all dissolved in water with considerable ease. 



