^4 CALCAREOUS MANURES— THEORY. 



length, and from which an extract will be translated and given in the ap- 

 pendix. The facts respecting humic acid, as concisely stated in the follow- 

 ing quotation from Professor Rennie, furnish strong confirmation of some 

 of the opinions which I have endeavored to maintain. It will however be 

 left, without farther comment, for the reader to observe the accordance, and 

 to make the application. 



" Humic add and humin. — In most chemical books the terms ulmic add 

 and ulmin are used, from ulmus, elm ; but, as its substance occurs in most, 

 if not all plants, the name is bad, I prefer Sprengel's terms, from humus, 

 soil. 



" This important substance was first discovered by Klaproth, in a sort of 

 gum from an elm ; but it has since been found by Berzelius in all barks; by 

 M. Braconnot in saw-dust, starch, and sugar ; and, what is still more in- 

 teresting for our present purpose, it has been found by Sprengel and M. 

 Polydore Boullay to constitute a leading principle in soils and manures. 

 Humin appears to be formed of carbon and hydrogen, and the humic acid 

 of humin and oxygen. Pure humin is of a deep blackish brown, without 

 taste or smell, and water dissolves it with great difficulty and in small quan- 

 tities; consequently it cannot, when pure, be available as food for plants. 



" Humic acid however, which, I may remark, is7iot sour to the taste,Yeadi]y 

 combines with many of the substances found in soils and manures, and not 

 only renders them, but itself also, easy to be dissolved in water, which in 

 their separate state could not take place. In this way humic add ivill com.- 

 bine toith lime, potass, and ammonia, in the form of humates, and the small- 

 est portion of these will render it soluble in ivater and Jit to be taken up by 

 the spongelets of the root fibres. 



" It appears to have been from ignorance of the important action of the 

 humic acid in thus helping to dissolve earthy matters, that the older writers 

 were so puzzled to discover how lime and potass got into plants ; and it 

 seems also to be this, chiefly, which is so vaguely treated of in the older 

 books, under the names of extractive, vegetable extract, mucilaginous mat- 

 ter, and the like. Saussure, for instance, filled a vessel with turf, and mois- 

 tened it thoroughly with pure water, when by putting ten thousand parts 

 of it by weight under a heavy press, and filtering and evaporating the fluid, 

 he obtained twenty-six parts of what he termed extract; from ten thou- 

 sand parts of well dunged and rich kitchen garden mould, he obtained ten 

 parts of extract ; and from ten thousand parts of good corn field mould, 

 he obtained four parts of extract. 



"M. Polydore Boullay found that the liquid manure, drained from dung- 

 hills, contains a large proportion of humic acid, which accounts for its fer- 

 tilizing properties so well known in China and on the continent ; and he 

 found it also in peat earth, and in varying proportions in all sorts of turf 

 It appears probable, from Gay-Lussac having found a similar acid, (techni- 

 cally a^umic add,) on decomposing the prussic acid, (technically hydro- 

 cyanic add,) that the humic acid may be found in animal blood, and if so, 

 it will account for its utility as a manure for vines, &c. Dobereiner found 

 the gallic acid convertible into the humic." 



When the last edition of this essay was published, (in 1835,) the above 

 annunciation had but just before been made, showing that there was indeed 

 high scientific authority for the very general existence of a vegetable acid 

 in soils. And since that time, the fact has been admitted by almost all 

 scientific writers, and has been treated of at length in sundry chemical works 

 and reports of geological surveys in this country. The doctrine of the 

 existence of an acid of soil, of vegetable origin, which before had scarcely any 

 other authority for its support than mine, humble and obscure as that was, 



