178 Bibliography. 



culturist and physiologist. Since the time of Sir Humphry Davy 

 no champion of agricultural chemis^try has before appeared, and this 

 science, without which no rational system of agriculture can be hoped 

 for, has been apparently neglected. 



Great stress has been laid, by chemists and vegetable physiologists, 

 on that constituent of soils which they have variously designated as hu- 

 mus, humin, coal of humus, humic acid, ulmin, extractive matter, geine, 

 soluble and insoluble, and apotheme. The modifications of humus, 

 which are soluble in alkalies, have been called humic acid, while those 

 which are insoluble have been described as humin, and coal of humus. 

 Berzelius, in 1833, published, in the memoirs of the Stockholm Acad- 

 emy,* an account of two new acids, the crenic and the apocrenic, found 

 in the waters of Porla well, in Sweden, and which he had previously 

 (1807)t designated in his examination of those waters, under the appel- 

 lation of extractive matter; and it will be seen by our notice of Dr. 

 Jackson's geological survey of Rhode Island, in this number, that he 

 has proved extensively the existence of these two acids in the soils of 

 that State, as well as in certain natural waters. It is this substance, 

 we repeat, by whatever name it is called, to which so much impor- 

 tance has been attached by writers on vegetable physiology, and by 

 agricultural chemists, as probably constituting an important part of 

 the food of plants. 



" The opinion that this substance is extracted from the soil by the 

 roots of plants, and that the carbon entering into its composition, 

 serves in some form or other to nourish their tissues, is so general, 

 and so firmly established, that hitherto any new argument in its favor 

 has been considered as superfluous ; the obvious difl^erence in the 

 growth of plants, according to the known abundance or scarcity of 

 humus in the soil, seemed to aflbrd incontestable proof of its correct- 

 ness. Yet this position, when submitted to a strict examination, is 

 found to be untenable, and it becomes evident, from the most conclu- 

 sive proofs, that humus, in the form in which it exists in the soil, does 

 not yield the smallest nourishment to plants." 



" The names given to these substances might lead to the supposi- 

 tion that their composition is identical. But a more erroneous notion 

 could not be entertained. Thus, humic acid, obtained by the action 

 of hydrate of potash on saw-dust, contains, according to the accurate 

 analysis of Peligot, 72 per cent, of carbon, while that from turf and 

 brown coal contains, according to Sprengel, only 58 per cent. ; that 

 produced by the action of dilute sulphuric acid on sugar, 57 per cent. ; 

 and that lastly which is obtained from sugar or from starch, by means 



* Kong. Vet. Acad. Had. 1833, p. 18. Poggendorff's Anaalen, xxix. 1, and 238. 

 Also, Thomson's Chemistry of Organic Vegetable Bodies, pp. 146, 1838. 

 t Af handlingar, p. 145. 



