SEPARATION OF THESE ORGANIC SUBSTANCES. 281 



bodies, — when once admitted, in consequence of their solubility, into the 

 circulating system of plants, — are converted into portions of their sub- 

 stance, and really do minister to their natural growth. 



Separation of these Organic Constituents. — 1°. When on washing 

 with hot water a soil imparts a colour to the solution, the liquid must be 

 filtered and evaporated, to perfect dryness. On treating with water 

 what remains after the evaporation, the humic acid and humates remain 

 insoluble, while the crenic and apocrenic acids are taken up by the wa- 

 ter along with the soluble saline matter which the soil may have con- 

 tained. By evaporating this second solution to perfect dryness, weigh- 

 ing the residue, and then heating it to dull redness in the air, the loss 

 will indicate something more than the quantity of these acids present in 

 the soil. By burning the dried insoluble matter, also, the quantity of 

 humic acid present in it may in like manner be determined. 



2°. After being washed with pure water, the soil is to be boiled with 

 a solution of carbonate of soda, repeated once or twice as long as a brown 

 solution, more or less dark, is obtained. Being filtered, and then ren- 

 dered sour by muriatic acid, brov/n flocks fall, which being collected on 

 the filter, perfectly dried and weighed, give the quantity of humic acid 

 in the soil. As this dry humic acid generally contains some earthy 

 matter, it is more correct to burn it, and to deduct the weight of the ash 

 which may be left. 



3°. The insoluble (coaly) humus still remains in the soil. On boiling 

 it now in a solution of caustic potash for a length of time, and till a fresh^ 

 solution ceases to become brown, the coaly humus is entirely dissolved — 

 being converted according to Sprengel into humic acid. The addition 

 of muriatic acid to this solution, till it has a sour taste, throws down the 

 humic acid in the form of brown flocks, which may be collected, dried, 

 and weighed as before. 



4°. If there be any mudesite of alumina in the soil, it is also dis- 

 solved by the potash, but is not thrown down when the solution is ren- 

 dered sour by muriatic acid. The entire w-eight of oi-ganic matter in the 

 soil being therefore determined by burning it in the air, after being 

 perfectly dried, the difference between this weight and the sum of those 

 of the humic acid and insoluble humus will be the proportion of the 

 other acids present. Thus, if, by burning in the air, the soil lose 6 per 

 cent., and give 2 per cent, of humic acid, and 2 of insoluble humus, there 

 remain 2 per cent, for other organic substances in the soil. 



In general, it is considered sufficient to ascertain only the whole loss 

 by burning, and the quantity taken up by carbonate of soda, the propor- 

 tion of the other substances present being in most cases so small as to be 

 capable of being precisely estimated by great precautions only. 



§ 2. On the exact chemical constitution of the earthy part of the soil. 



In reference to the general origin of soils — to their geological rela - 

 tions — and to the simplest mode of classifying them, — I have shown yoa 

 that the earthy part of nearly all soils consists essentially of sand, clay, 

 and lime (p. 230). But in reference to their chemical relations to the 

 plants which grow, or may be made to grow, upon them, it is necessary, 

 as you are ^w aware, to take a more refined and exact view of their 



