Elements of agricultural chemistry, in a course of lectures of the Board of Agriculture / By Sir Humphry Davy.

  • Davy, Humphry, Sir, 1778-1829.
Date:
1846
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Licence: Public Domain Mark

Credit: Elements of agricultural chemistry, in a course of lectures of the Board of Agriculture / By Sir Humphry Davy. Source: Wellcome Collection.

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    another portion of acid is brought out, and so on, till the carbonate is entirely decomposed. The gas escapes through the tube, while any watery vapour that rises along with it is retained by the chloride of calcium. Heat is now applied to the flask, to expel any carbonic acid that remains in the solution; and by raising a quantity of steam, the gas that filled the apparatus is also driven off. When the operator possesses an air-pump, it is simpler, after the action of the acid has ceased, to place the apparatus under a receiver and exhaust the air; readmitting the air, however, through another chloride of calcium tube. A more convenient form of the apparatus is to have the small tube leading from the flask to the chloride of calcium tube twice bent, so that the latter stands horizon- tally over the top of the flask. When the gas has been entirely expelled, the loss of weight is ascer- tained ; it represents the carbonic acid of the carbonates present. Statement of Results.—The quantitative analysis of the soluble saline matter being concluded, it is necessary, in stating the results, to indicate the state of combination in which the various bodies found existed in the solution; but as the remarks on this subject apply also to the statement of the remaining parts of the analysis, they are deferred till an account of the subsequent processes has been given. ANALYSIS OF THE FINELY-DIVIDED MATTER OF THE SOIL INSOLUBLE IN WATER. From the previous examination of the soluble matter of the soil, and particularly of the grit and sand, a good deal of information regarding the nature of the impalpable or finely-divided portion will have been obtained. The impalpable portion of a fertile soil consists of minerals in fine division of parts, more or less decomposed, mixed with organic mat- ter, chiefly vegetable, in a state of decay. The organic matter may, and often does, demand a separate examination, chiefly with the view of as- certaining its condition; whether it is in an inert condition, as is the case with peat, or undergoing steady, though slow decay, or in any of the numerous and little investigated intermediates states. Very little accurate data yet exists on which to base such an examination; and hence, it is usual merely to estimate the amount of organic matter, or the matter destructible by a low red heat. With respect to the inorganic matter, much less difficulty exists. After destroying the organic matter, the residue will generally be found to consist of silica and silicates ; of car- bonates, as of lime and magnesia, that existed as such in the soil; of peroxide of iron and alumina, resultingfrom decomposed silicates; and of the bases that were united with the organic matters. It is desirable to ascer- tain the percentage of carbonate of lime in a soil, as it is known to be essential to fertility; and the same may be said of carbonate of magnesia and peroxide of iron. For this purpose, as was recommended in the mechanical analysis, the impalpable matter, after ignition, is treated with cold diluted muriatic acid, and the lime, and if necessary the magnesia and peroxide of iron are estimated. The portion left undissolved, by the diluted muriatic acid is then analysed; and the method usually followed, is to treat it first with boiling concentrated muriatic acid, and next with hot concentrated sulphuric acid; regarding the residue as silica, and diffi- cultly decomposable silicates. In place of this method, however, it is shorter, at once to decompose the residue left by the diluted muriatic acid by ignition with carbonate of potash, only, the alkalies cannot, in this way, be estimated. Still, however, it is preferable to make a separate