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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    or the whole may be mixed together and concentrated by evaporation in a porcelain capsule; but either the concentration should not be carried so far as to produce a deposit from the solution, or the deposited matter should be separated by filtration, and separately examined. When the solution, as is often the case, contains but little organic matter, the testing for soluble saline substances may be immediately proceeded with; for although organic matter does interfere with the action of some of the reagents to be employed, when it is present only in small quantity, this interference will not be of great consequence. When, however, the dark colour of the solution indicates the presence of much organic matter, it is necessary to destroy it before proceeding with the testing. For this purpose, the solution is evaporated in a porcelain capsule, till it becomes of syrupy consistence; after which, it is transferred to a platinum crucible and evaporated to perfect dryness. The crucible is then heated to in- cipient redness, and continued in that state till all organic matter is burnt off, and the contents lose their blackness. It is to be kept in mind, how- ever, that if the solution contained volatile substances, such as nitric acid or ammoniacal salts, they are entirely dissipated in this process, and must be tested for in a portion of the solution reserved for the purpose, and not evaporated to dryness and ignited. It is also to be ob- served, that the heat is kept as low as possible during the burning off of the organic matter, to avoid, as far as it can be done, the loss of chlorides, which are almost invariably present, and are liable to be volatilised at high temperatures. After the organic matter is destroyed, the residuum is treated with dis- tilled water, to which a little nitric acid has been added; and if a small portion of insoluble matter still remains, it is silica, and may be separated by filtration. The slightly acid solution is now rendered exactly neutral by ammonia, or as nearly so as can be effected without causing precipita- tion; and thus prepared, the saline solution, which contained much organic matter, is ready for being tested. The operation of testing is carried on in wine glasses, or small conical test glasses, (Fig. 29,) un- less otherwise specified. For each trial, a small quantity of the solution (about 60 grains) is taken, and the reagent is added in quantity corresponding to its strength, and to the concentration of the solution. After the addition of the test, the mix- ture is shaken, or stirred with a glass rod. Trial of Acidity or Alkalinity.—This is to be done by dipping a grey litmus-paper into that portion of the solution first obtained, for the more diluted portions cannot be expected to give a decided reaction. The solution obtained, in cases where it has been found necessary to destroy the organic matter, must, of course, be acid, from the nitric acid used in the process. The most useful litmus paper is prepared by staining the surface of perfectly pure drawing paper, (Harding’s), with an infusion of litmus, to which as much nitric acid has been added, as to change its blue to neutral-tint. If the colour of the neutral or grey tint is changed to red, the indication is acid, if to blue, it is alkaline. The saline solu- tion is generally neutral, but occasionally otherwise. Detection of Nitric Acid.—It is very seldom, indeed, that soils in this