1295. The doctrine of humus in the form of humic acid being one of the most important parts of the food of plants was first broached in the beginning of the present century, and it was eagerly received, as it seemed to reconcile theory with practice, and to account for the advantage which practical men found in using stable manure in a putrescent state:-'In 1802, the celebrated Klaproth received from Palermo a substance which exuded spontaneously from the bark of a species of elm, and to which Dr. Thomson gave the temporary name of ulmin. It dissolves speedily in a small quantity of water, in which respect it is like a gum; but when the solution is very much concentrated by evaporation, it is not in the least mucilaginous or ropy, nor does it answer as a paste, and in this respect it differs essentially from gum. When a few drops of nitric or oxymuriatic acid are added to the solution, it becomes a gelatinous mass, which, when slowly evaporated to dryness, and heated with alcohol and again evaporated, leaves a light brown bitter and sharp resinous substance. Thus it appears that ulmin, by the addition of a little oxygen, is converted into a resinous substance. In this new state it is insoluble in water. This property is very singular. That a substance soluble in water should assume the resinous form with such facility, is very remarkable. (Thomson's Chemistry, vol. iv. p. 696.) Berzelius has found this curious substance in all barks; Braconnot in sawdust, starch, and sugar. But, what is more to our purpose, Sprengel and Polydore Boullay have found it to constitute a leading principle in all soils and manures. Sprengel appropriately calls it humin from its existence in all soils, ulmin being given to it by Dr. Thomson as a temporary name.' (Quart. Jour. Agr., vol. iv. p. 620.)
