52 



Essay on the Analysis of Soils, 



tube No. 1 will consist of very fine particles of sanely clay^ and 

 perhaps carbonate of lime. The sand will appear deposited in 

 the bottom of the tube. The clay may be easily diffused in the 

 water above it by stirring it carefully with a small rod without 

 reaching the sand. It may then be decanted off with the water 

 into another tube (No. 3), and allowed to settle : this part of the 

 operation may be carried to a great degree of perfection by great 

 care^ and by examining the results occasionally with a small mi- 

 croscope ; but for all common practical purposes it is sufficient to 

 separate the vegetable earth from the mineral^ and the visible par- 

 ticles of sand from the finer. 



The contents of No. 1^ ha\dng been collected, as well as those of 

 No. 3, are dried over the fire and accurately weighed. The same 

 is done with the earths which remain on the sieves. 



All the water in which the earths have been diffused and washed 

 is collected and passed through filtering-paper, and then set over 

 the fire in a common saucepan. It is boiled away gently until it 

 is reduced to a small portion, which begins to look turbid. The 

 complete evaporation is finished in an evaporating-dish, as slowly 

 as possible, and the residue is the soluble matter contained in the 

 soil. It will be sufficient to dry and weigh this, as its further 

 analysis would require more skill and chemical knowledge than 

 we suppose in the operator. Salts may be detected by the taste, 

 or by the crystals formed in the evaporation ; but, unless there is 

 a decided saline taste, the whole may be considered as soluble 

 humus, and the immediate fertility of the soil depends greatly on 

 the quantity of it. 



To recapitulate what has been obtained, — we shall have the 

 coarse grit in sieve No. 1 ; the sand in Nos. 2 and 3 ; the fine 

 earth separated in the tubes Nos. 1 and 3 ; the humus in tube No. 2 

 and on the filtering-paper ; and the soluble parts in the evapo- 

 ratinof-dish. All these substances must be well dried over the 

 fire, as was done with the soil at first, and each separate part 



stances apart, and which appears in various forms, according to the degree 

 of decomposition, it has undergone and the circumstances under which it has 

 taken place, may be obtained by dissolving it in a caustic alkali, and preci- 

 pitating it by means of an acid. The precipitate has been named humic- 

 acid, or ulmic-acid, and has often been confounded with humus, or vege- 

 table mould. It is no doubt a component part of humus ; but it. is not found 

 pure and uncombined in the earth, as far as we know. The real humus is 

 a very compound substance, and exists in so many forms, that the experi- 

 ments, which have hitherto been made, have not much increased our know- 

 ledge of it. It would be worthy the labour of some of our greatest 

 chemists to trace the progress of vegetable decomposition, under^ various cir- 

 cumstances, and to detect the regular change which takes place in the 

 arrangement of the elementary component parts — carbon, oxygen, hydro- 

 gen, and occasionally nitrogen — as the living vegetable dies, and is gradually 

 transformed into humus, when deposited in the earth. 



