By deducting the weight of this residue from that of the air-dried soil employed, and subtracting from the result the amount of water known to be present in the air-dried soil, we ascertain the total quantity of the soluble ingredients.

It is well to control this result by evaporating the filtrate and washings, first, in a porcelain basin upon a sand-bath, then, in a weighed platinum capsule, placed on a water-bath, to dryness, and heating the residue in a water-oven till its weight is constant. The total quantity of soluble matter is then calculated for 100 parts of the air-dried soil.

Determination of the amount of Organic Matter soluble in Water.-The residue obtained in the above experiment is gently ignited till all organic matter is burnt off. The loss of weight indicates the amount of soluble organic matter present.

Determination of the Individual Constituents soluble in Water.-(Should a large amount of organic matter be present, it is better to destroy it by moderately heating the weighed portion of the soil, before proceeding with the subsequent operations.)

An accurately weighed1 quantity of the soil (which must be varied according to the number of constituents to be determined in the solution, about 1500 grains being usually sufficient) is introduced into a beaker, and treated with water exactly as directed for the determination of the total amount of solid matter, except that the residue may be collected on an ordinary (not weighed) filter.

The solution and washings are evaporated till they do not measure more than 10 or 12 ounces, introduced into a dry weighed flask, and accurately weighed. Determination of the Buses (except Alkalies).—(It very rarely happens that the aqueous solution contains a sufficient quantity of alumina, manganese, iron, or of silica or phosphoric acid, for quantitative estimation; should this, however, be the case, their separation and determination may be effected by the ordinary methods.)

About one-fifth of the aqueous solution is poured into a beaker, and the flask afterwards accurately weighed to ascertain the amount of solution which has been taken.

The lime is then determined, in the usual manner, as oxalate, and, subsequently, the magnesia in the filtrate, as pyrophosphate.

The amounts of lime and magnesia are calculated for the whole of the solu tion, and thence, for 100 parts of the air-dried soil.

Determination of the Alkalies.-The alkalies are determined in two-fifths of the aqueous solution by the method given for their determination in waters, p.

631.

Determination of Sulphuric Acid.-The sulphuric acid is determined in another fifth of the aqueous solution, as sulphate of baryta.

Determination of Chlorine.-The remaining fifth of the solution is employed for the estimation of chlorine as chloride of silver.

Determination of the Constituents soluble in Hydrochloric Acid.-The residue left on boiling the weighed portion of soil with water is dried on the filter by exposure in a warm situation, detached, as far as possible, from the filter, well powdered, and dried, for an hour or two, at 212° F.

About 500 grains of this residue (accurately weighed) are introduced into a platinum dish, and gently ignited till all organic matter is perfectly destroyed, and the mass completely incinerated. The residue is then transferred to a large flask, covered with water, and concentrated hydrochloric acid gradually added till all effervescence has ceased; a little more hydrochloric acid is then added, the mixture boiled for about an hour, upon a sand-bath, allowed to subside, the

The soil may be conveniently weighed in a light beaker.

clear liquid carefully poured off into a beaker, and the residue once more boiled, for a few minutes, with a fresh portion of hydrochloric acid. The clear liquid is again decanted, mixed with the first portion, the whole diluted with water, and filtered through a weighed filter. The residue in the flask is thrown on to the filter, and washed till the washings have no longer an acid reaction.

The solution and washings are mixed with a little nitric acid, and carefully evaporated to dryness, in an air-bath, the residue moistened with concentrated hydrochloric acid, then digested with dilute hydrochloric acid for some time. The undissolved silica is collected upon a filter, and determined in the usual manner (p. 594).

The filtrate and washings are concentrated by evaporation, introduced into a stoppered bottle, and accurately weighed.

The amount of silica obtained is calculated for the quantity of aqueous residue employed, and subsequently, for the quantity of this residue which is known to correspond to 100 parts of the air-dried soil.

Determination of the Bases (except the Alkalies), and of Phosphoric Acid.— About one-half of the hydrochloric solution (accurately weighed), is gradually mixed with ammonia, with constant stirring, until a precipitate begins to appear, which does not redissolve entirely on stirring; a gentle heat is then applied, and when the precipitate has redissolved, some acetate of potassa is added until the liquid has a distinct red color (should there be not enough iron for this purpose, a little sesquichloride of iron should be added1). The mixture is introduced into a capacious porcelain dish, and boiled till the liquid portion has lost its red color, denoting that all the acetate of sesquioxide of iron is decomposed; the solution is then filtered while hot, and the precipitate washed with hot water.

The precipitate, which contains alumina, sesquioxide of iron, and phosphoric acid, is dissolved off the filter in warm dilute hydrochloric acid (the filter being well washed); the solution is mixed with ammonia till a permanent precipitate begins to form, and sulphide of ammonium added in excess. The mixture is digested for an hour or two at a moderate heat, the precipitate (sulphide of iron and, perhaps, of manganese and alumina, with a little phosphoric acid) filtered off and washed with water containing sulphide of ammonium.

The filtrate and washings (containing most of the phosphoric acid) are evaporated to a small bulk, acidulated with hydrochloric acid, and heated with a little concentrated nitric acid to oxidize and dissolve the separated sulphur. The phosphoric acid is then determined in the solution as basic perphosphate of iron (p. 593).

The precipitate, containing sulphide of iron, and perhaps, of manganese and alumina, is dissolved in hydrochloric acid, the solution heated till all sulphuretted hydrogen is expelled, boiled with a little nitric acid, and mixed with the acetic solution originally filtered off from the precipitate containing phosphoric acid. The solution is concentrated, if necessary, by evaporation, mixed with chloride of ammonium and excess of ammonia, heated, and filtered.

The lime and magnesia in the filtrate are determined as usual.

The precipitate, which contains alumina, iron, and manganese, is treated exactly as in the analysis of clays (p. 605); the ignited precipitate, containing only iron and manganese, is subsequently dissolved in hydrochloric acid, and the iron and manganese separated and determined by the method employed in the analysis of cast-iron (p. 626).

The results are all calculated upon 100 parts of air-dried soil, the iron and manganese being calculated as peroxides.

Determination of Alkalies.-About one-fourth of the hydrochloric solution is

In this case, which seldom occurs, the iron must of course be determined in another portion.

evaporated to dryness in an air-bath, the residue stirred, and heated with water, an excess of baryta-water added, the solution again heated for some time, and filtered; the filtrate is then treated as directed for the determination of alkalies in the aqueous solution.

The alkalies are calculated for 100 parts of air-dried soil.

Determination of Sulphuric Acid. This acid will only occur in the hydrochloric solution when the washing of the aqueous residue has been incomplete. It is determined as sulphate of baryta.

Analysis of the portion insoluble in Water and Acids.-This residue is dried in the water-bath, till its weight is constant.

Determination of Silicic Acid and of all Bases except Alkalies. This is conducted according to the method described in the analysis of clays (p. 605); about 15 or 20 grains of the residue are fused with carbonate of potassa and soda.

Determination of Alkalies.-About 15 or 20 grains of the residue are ignited with three or four parts of hydrate of baryta, and the alkalies estimated as in the analysis of glass (p. 624).

The results of the analysis of the insoluble residue are also calculated upon 100 parts of the air-dried soil.

In calculating the results of the analysis of a soil, the constituents of the aqueous solution should be arranged upon the same principles as in the analysis of waters.

The whole of the lime and magnesia in the portion soluble in hydrochloric acid will exist as carbonates, unless sulphuric acid be present, when it will exist as sulphate of lime. The determinations of the carbonic acid, and of lime and magnesia, will therefore usually check each other.

If there be an excess of carbonic acid, it must exist as carbonate of iron or manganese. Unless good reason is seen for a different assumption, the iron and manganese are calculated as sesquioxide and binoxide, respectively.

There are evidently several means of control in the analysis of a soil.

The amount of the constituents soluble in water, found by direct determination, should agree with that determined by weighing the residue, and with the joint amount of the various substances estimated in the solution.

The sum of the weights of the substances determined in the hydrochloric solution (plus the carbonic acid) should coincide (or nearly so) with the amount ascertained to be soluble in the acid.

In this calculation, it must not be forgotten that the organic matter has been destroyed before dissolving the residue in hydrochloric acid, so that its weight must be added in with the substances determined in the solution.

Other methods of control will readily suggest themselves to the analyst.

ANALYSIS OF THE ASHES OF VEGETABLE SUBSTANCES.

§ 443. It would be very desirable to ascertain the exact forms of combination in which the mineral constituents exist in vegetable tissues, but, up to the present time, it has not been found possible to determine these without previously destroying the organic matters with which they are associated; and since we can never be sure that the processes employed for this purpose do not modify or decompose the saline combinations, the information thus acquired can only possess a certain relative value, enabling us to compare, with considerable accuracy, the mineral ingredients of plants with those of other plants, or of soils, but not to be regarded in the light of an actual account of the constitution of the mineral portion of the plant.

Much attention has been bestowed by analytical chemists upon the preparation of the ashes of vegetable substances for analysis, so that they may suffer the least possible alteration, but nearly every method which has at present been used for this purpose, is based upon the destruction of the organic matter by heat.

When we remember that vegetable tissues generally contain compounds of the alkalies and alkaline earths with organic acids, together with silicic acid, sulphates, phosphates, and chlorides, and that, in the destruction of the organic matter by heat, these substances are exposed to a high temperature in contact with charcoal and moisture, we are not surprised that a considerable alteration should be effected in these constituents by the incineration.

Since the old method of preparing the ashes of vegetables is by far the simplest and most easy of execution, and has not yet been fairly superseded by any other which lays claim to very superior accuracy, we shall here consider the analysis of ashes prepared by that method, merely suggesting, that if all ashanalyses were carefully executed by one standard method, the purposes of comparison would be fully answered.

INCINERATION OF VEGETABLE SUBSTANCES.

The portion of the plant intended for incineration must be thoroughly cleansed with a dry cloth.

Roots and woods may be cut up into slices or fragments, and dried in an oven. Fruits are also cut into slices and dried in the same way. Seeds, leaves, and flowers are simply dried. [Stalks, straw, &c. may be conveniently dried, laid in bunches, and kindled at a flame, the ash being allowed to fall into a dish, and the incineration subsequently completed in the ordinary way. Or, the stalks, &c. may be cut into small pieces and incinerated.]

The substance, thus prepared, is placed in an earthen crucible, or in a porcelain dish, and gradually heated over a charcoal fire. As soon as the matter is thoroughly carbonized, and no more fumes are evolved, the heat is somewhat raised, and continued for a considerable period (several hours are sometimes necessary), until all or the greater part of the carbon has burnt off; the heat must not be sufficient to fuse the ash.

If a large quantity of carbon still remains unburnt, the incineration is carried further in a platinum capsule, over a gas-burner, the ashes being stirred at intervals, with a platinum spatula.

When all or the greater part of the carbon is consumed, so that the ashes appear white or light gray, a small specimen is taken out and treated with hydrochloric acid; if any hydrosulphuric acid is evolved, the ash must be mixed (in a mortar) with a small quantity of pure red oxide of mercury (which volatilizes without residue) and heated in the platinum dish (covered) until all the oxide is volatilized. (A portion of the ash is boiled with dilute nitric acid, filtered, and tested with hydrosulphuric acid.) In this way, any sulphide, arising from the reduction of the sulphates by the charcoal, is oxidized and reconverted into sulphate.

The ash is well mixed and preserved in a stoppered bottle.

Determination of the total amount of Ash.-A quantity of the vegetable substance varying with the amount of ash which it is likely to yield, is dried in the water oven until its weight is constant. It is then very carefully incinerated in a platinum crucible, till nearly the whole of the carbon has burnt off. A quantity of pure red oxide of mercury is then mixed with the ash in the crucible, and the mixture very strongly ignited. The ash is afterwards moistened with carbonate of ammonia, thoroughly dried in an air-bath, and weighed.

In very accurate determinations, these operations should be repeated until the weight is constant.

QUALITATIVE ANALYSIS.

The following constituents are usually found in the ashes of plants:

In marine plants, also, bromine and iodine should be sought.
Fluorine is occasionally found in ashes of plants.

The ashes also usually contain more or less unconsumed carbon, and some sand mechanically adhering to the plant.

A portion of the ash is boiled with water, and filtered.

The aqueous solution is tested in the usual manner, for the bases and acids mentioned above.

The residue insoluble in water is washed and boiled with dilute hydrochloric acid. Effervescence of course indicates carbonic acid, which should not be accompanied by hydrosulphuric acid, if the preparation of the ash has been properly effected. The acid solution is examined, as usual, for the above bases and acids (except carbonic and hydrochloric).

The portion insoluble in hydrochloric acid (which most frequently consists of sand mechanically adhering to the plant, and of unconsumed carbon), is washed, dried, ignited, if necessary, to burn off the carbon, and, should it be deemed requisite, analyzed, as usual, by Table VIII.

It is only, however, in the case of ashes very rich in silicic acid (equisetaceæ, &c.) that it is necessary to examine this residue.

QUANTITATIVE ANALYSIS.

It is rarely necessary, except for some special purpose, to analyze the aqueous solution separately.

Determination of Carbonic Acid.-About 30 or 40 grains of ash are employed for the estimation of the carbonic acid according to the method of Fresenius and Will (p. 617).

Determination of Chlorine.-15 or 20 grains of the ash are boiled in a flask, with dilute nitric acid (the acid being gradually added to water previously poured over the ash), and the chlorine determined, in the filtered solution, as chloride of silver.

Determination of the Remaining Constituents. (The manganese and alumina are very rarely determined in ashes).-About 100 grs. of ash are introduced into a flask, covered with water, and hydrochloric acid gradually added in considerable excess. The contents of the flask are then boiled for some time, thoroughly rinsed out into a porcelain dish, and evaporated to dryness in the air-bath. The residue is treated, as usual, with dilute hydrochloric acid, the insoluble portion (silicic acid, sand, and charcoal) collected on a weighed filter containing very little ash, washed till the washings have no acid reaction, and dried.

The acid filtrate and washings are evaporated to a small bulk, and accurately weighed in a stoppered bottle.

Determination of Sulphuric Acid.-About of the solution is employed for the determination of sulphuric acid as sulphate of baryta.