30 OP THK ORGANIC MATTER PRESENT IN THE SOIL. [AppCTldlXf 



separated from it,* when it is to be dried at 250° F., till it ceases to lose weight 

 Tlie final weight, minus that of the filter, gives the quantity of humic acid con- 

 tained in the portion of soil submitted to examination. As it is rarely possible to 

 wash the humic acid perfectly upon the filter, rigorous accuracy requires that the 

 filter and acid should be burned after being weighe;!, and the weight of ash left, 

 minus the known weight of ash left by the filter,t deducted from that of tlie 

 acid as previously determined. It is to be observed here that by this, which 

 is I'eally the only available method we possess of estimating; the humic acid, a 

 certain amount of loss arises from its no«. being wholly insoluble, the acid 

 liquid which passes through the filter being always more or less of a brown 

 colour.: 



11°. Determination of the insoluble hunniis. — Many soils after this treatment 

 with carbonate of soda ai-e still more or less of a brown colour, evidently due 

 to the presence of other organic matter. To separate this, Sprengel recom- 

 mends to boil the soil, which has been treated with carbonate of soda, and 

 which we suppose still to remain in the flask, with a solution of caustic potash, 

 repeated, if necessary, as in the case of the soda solution. By this boiling, 

 the vegetable matter, which was insoluble in the carbonate of soda, is changed 

 in constitution and dissolves in the caustic potash, giving a brown solution, 

 from which it may be separated in brown flocks by the addition of muriatic 

 acid, and then collected and weighed as above described. 



In some soils, also, distinct portions of vegetable fibre, sucfi as portions of 

 roots, &c., are present, and may be separated, mechanically dried, and weighed. 



12'^. Of other organk sidstatices present in the soil. — The sum of the weights 

 of the above substances deducted from the whole weight of organic matter, as 

 determined by burning, gives that of othr.r organic substances present in the 

 soil. The quantity of these is in general comparatively small, and, unless they 

 are soluble in water, there is no easy method of separating them, and determin- 

 ing their weight. The following two methods, however, may be resorted to: — 



1*^. Half a pound or more of the moist soil may be boiled with two separate 

 pints of distilled water, the liquid filtered and evaporated to a small bulk. From 

 clay soils, when thus boiled with water, the fine particles do not readily subside. 

 Sometimes, after standing for several days, the water is still muddy, and passes 

 muddy through the filter, but, after being evaporated, as above recommended, to 

 a small bulk, most of the fine clayey matter remains on the paper when it is 

 again filtered. As soon as it has thus passed through clear, the liquid may be 

 evaporated to perfect dryness at 2,50" F., and weighed. Being now treated 

 v.nth water — a portion will be dissolved — this must be poured off, and the inso- 

 luble remainder again perfectly dried and weighed. If this remainder be now 

 heated to redness in the air, any organic matter it contains will be burned off, 

 and its weight ascertained by the loss on again weighing. I'his loss may be 

 considered as humic acid rendered insoluble by drying.§ It does not require to 

 be added to the weight of humic acid already determined (lO'^), because in 

 that experiment a portion of soil was employed which had not been boded in 

 water., and from which therefore the carbonate of soda would at once extract 

 all the humic acid. The present experiment need only be made when it is de- 



* This is ascertained by collecting a few drops of what is passing through upon a piece of 

 clean glass or platinum, and drying them over the lamp, when, if a perceptible stain or spot is 

 left, the substance is not sufficiently washed. 



t The ash left by the paper employed for filters should always be known. This is ascer- 

 tained, once for all, by drying a quantity of it in the way described in the previous note, 

 weighing it in this dry state, burning it, and again weighing the ash that is left. In good 

 filtering paper, the ash ought not to exceed one per cent. 



X The portion which thus remains in the solution may be precipitated by adding a small 

 quantity of a solution of alum, and afterwards pouring in ammonia in excess. The alumina 

 falls coloured by the organic matter, and after being colleiited on a filter, washed, and dried, 

 the weight of organic matter in the precipitate may be determined approximately as des- 

 cribed under 12° (2°). 



§ See Lecture xiii., § I. 



