If the weight of the carbon dioxid dissolved in V′ cubic centimeters of the liquid in the evolution flask be represented by q, the coefficient of the solubility of pure carbon dioxid in this liquid will be, according to the law of the solubility of a gas, equal to k = q
V′a

The volume of k has been determined for various strengths of the sodium carbonate solution, using five cubic centimeters of hydrochloric acid containing 1.6 grams pure hydrochloric acid. For solutions disengaging from five to fifty milligrams of carbon dioxid, the mean value of k was found to be 1.8 milligrams in the absence of calcium chlorid. When calcium chlorid was present in quantities varying from 0.03 to 0.07 gram per cubic centimeter of liquid in the evolution flask, the value of k was 1.4 milligrams.

By adopting, according to circumstances, the one or the other of the above numbers and multiplying it by Va, as determined by experiment, results are obtained differing only 0.2 to 0.3 milligram from those secured by direct weighing of the evolved gas.

Dietrich^[208] has called attention to the necessity of adding the volume of the dissolved gas to the measured volume in such determinations, and this volume or weight is easily determined by the above formulas.

322. Belgian Method.—The method pursued at the Gembloux Station^[209] consists in taking from five to fifty grams of the sample of soil, according to its content in carbonate, rubbing it up in a porcelain dish with distilled water in order to make a thin paste. The mass is worked to drive out all the air, the whole washed into a flask of 300 cubic centimeters capacity, and the amount of carbon dioxid estimated by setting free with an acid, and collecting the carbon dioxid evolved in potash bulbs.

DIGESTION OF SOILS WITH SOLVENTS.

323. General Considerations.—There are two points in connection with the determination of mineral matters in the soil which must always be kept in view; viz., first, the estimation of the total quantities of material in the soil, and second, the study of those materials which are more easily brought into solution and thus made available for the food of plants.

It is well understood that the soil particles do not give up entirely to the plant the food materials which they contain. The practical value therefore of an analysis of a soil depends more upon the exact determination of the plant food available than upon its total quantity. From a mineral and geological point of view, on the other hand, an idea of the total composition of the soil is the object to be attained.

For the determination of the available plant food, various solvents have been proposed, none of which, perhaps, imitates very accurately the natural solvent action of organic life and moisture on the soil materials. A description of the standard methods of preparing soil extracts will be the subject of a few succeeding paragraphs.

324. Estimation of the Quantity of Materials Soluble in Water.^[210]—Five hundred grams of the air-dried soil are treated in a flask with 1,500 cubic centimeters of water, less the quantity of water already contained in the air-dried soil, which is volatile at 125°. The mass is frequently shaken and, after seventy-two hours, 750 cubic centimeters of the liquid filtered. The filtrate is evaporated to dryness in a platinum dish, dried at 120° and weighed. This is then incinerated and, after treatment with ammonium carbonate and gentle ignition, is again weighed. The further examination of the residue for acids and bases is made by some of the methods hereafter described.

325. Treatment with Water Saturated with Carbon Dioxid.—Two thousand five hundred grams of the air-dried soil are treated with 8000 cubic centimeters of distilled, and afterwards with 2000 cubic centimeters of water, which have previously, at room temperature, been saturated with carbon dioxid. The mixture is left in a closed flask for seven days, frequently shaken, after which 7,500 cubic centimeters of the liquid are filtered. The clear filtrate, after treatment with a little hydrochloric acid and a few drops of nitric acid, is evaporated to dryness. After the separation of the silica the traces of iron, alumina, lime, sulfuric acid, magnesia, potash, and soda, are estimated in the liquid in the manner hereinafter to be described. Phosphoric acid is always present in such a case, in such small quantities as to make its estimation unnecessary.

326. Treatment with Water Containing Ammonium Chlorid.—In the flask containing the residue from the last experiment; viz., the soil with 2,500 cubic centimeters of liquid, are added 1,500 cubic centimeters of water saturated with carbon dioxid, and 8,000 cubic centimeters of pure water in which five grams of ammonium chlorid are dissolved. The mixture is then left for seven days, with frequent shaking, and 7,500 cubic centimeters of the liquid are then filtered, and the substances dissolved, determined in the filtrate. In addition to the usual quantities of lime and magnesia, from two to four times as much alkali is dissolved by this treatment as is found in the solution from the water containing carbon dioxid alone.

327. Treatment with Water Containing Acetic Acid.—The acetic acid should be of such a strength that after it has fully acted on the soil it should still contain twenty per cent of free acid. 1000 grams of the soil dried at 100° are taken and the acid added in proper proportions and treated in the manner to be described for determining the solvent action of hydrochloric acid.

328. Treatment with Citric Acid Solution.—In ascertaining the quantities of soil materials soluble in a solution of citric acid, Dyer^[211] recommends the use of a carefully prepared citric acid solution. The digestion is carried on as follows: Place in a flask or bottle, holding about three liters, 200 grams of air-dried soil and two liters of distilled water, in which are dissolved twenty grams of pure citric acid. The soil is left, at room temperature, in contact with the one per cent acid for seven days, with thorough shaking several times a day. At the end of the digestion the solution is filtered and 500 cubic centimeters of the filtrate, corresponding to fifty grams of the soil, are taken for analysis for each ingredient to be determined.

The digestion in citric acid is especially recommended by Dyer because of its supposed near resemblance to the methods of solution of plant food practiced by the rootlets of plants. It is evident, however, that this process is in no sense an imitation of natural methods. The solution is to be used exclusively for the estimation of potash and phosphoric acid. Dyer concludes, from a comparison of the action of a solution of citric acid on soils of known fertility, that when as little as 0.01 per cent of phosphoric acid is dissolved from a soil by this treatment it is justifiable to assume that it stands in immediate need of phosphatic manure. The methods used by Dyer to determine the phosphoric acid and potash in the citric acid solution will be given in their appropriate place.

329. Treatment with Hydrochloric Acid.—The solutions of soils usually subjected to chemical analysis are those obtained by long treatment with hot mineral acids, among which the most common is hydrochloric.

It has long been assumed by soil analysts, perhaps not with justness, that such treatment removed from the soil, all those elements of plant food which could possibly be available for the needs of the growing crop. In this connection, however, the analyst must not forget that nature, in a series of years, with her own methods may easily accomplish what he in five days, even with the help of a hot mineral acid, may not be able to secure. Since, however, this method of solution has been so long practiced it is not the place here to throw doubt on its effectiveness without being able to suggest a better way. Of the mineral acids available no one possesses solvent powers for soils in a higher degree than hydrochloric. A somewhat detailed description will therefore be given of the methods of its use.

330. Strength of Acid to be Employed.—The fact that hydrochloric acid of nearly constant strength; viz., specific gravity 1.115, equivalent to 22.9 per cent hydrochloric acid, may be obtained by distillation, led Owen to use acid of this density in his classic work on soil analysis. Hilgard has lately reviewed the conditions of constant strength in the solvent with results confirming the statements of Owen.^[212] He evaporated on a steam-bath, to one-half its bulk, fifty cubic centimeters of hydrochloric acid, specific gravity 1.116, obtained by using the distillate from a stronger acid after rejecting the first third. The same operation was conducted with similar acid diluted with ten per cent of water. The acid used contained 22.96 per cent hydrochloric acid. The residual acid contained 21.49 per cent hydrochloric acid. These results lead Hilgard to believe that the changes arising from evaporation in hydrochloric acid during soil digestion are insignificant, compared with those due to its action on the soluble matters, and that evaporation during digestion is effective in maintaining a definite strength in the solvent. For this reason it is contended that evaporation in a porcelain beaker covered by a watch-glass is more effective in constancy of conditions than digestion in a closed flask under pressure.

331. Influence of Time of Digestion and Strength of Acid.—Loughridge has made an interesting study of the influence of the strength of acid and time of digestion on the extraction of soils.^[213] The method of preparing the soil for the determination of the above points is as follows:

The soil, having been passed through the appropriate number of sieves to obtain the fine earth is pulverized with a wooden pestle and thoroughly mixed. The hygroscopic moisture is determined, after exposing it in a place saturated with vapor, in a layer not exceeding one millimeter in thickness for twelve hours, and subsequently drying at 200° in a paraffin-bath. Of this dried substance, from two to three grams are used in the general analysis, the methods employed being in general those adopted by Peter.^[214]

The quantities of materials dissolved by acids of different densities are shown below. The determinations were made by methods hereafter to be described.

From the above table it is seen that the strongest acid exerts the least soluble effect upon the substances present in the soil, while the greatest degree of solution was obtained by the acid of 1.115 specific gravity. This result indicates that while lime and magnesia are probably present chiefly as carbonates, potash as well as alumina, and to some extent lime, are present as silicates, and for that reason are not as fully extracted by acid of low strength as by that of medium concentration.

In regard to the influence of the time of digestion, the acid of specific gravity 1.115 being used, the data obtained are given in the following table:

From this table it appears that the amount of dissolved ingredients increases up to the fifth day, the increase becoming, however, very slow as that limit is approached. It is also found that the ingredients offering the greatest resistance to this action are the same as those whose amounts were sensibly affected by the strength of the acid; namely, silica, potash, and alumina.

In regard to lime and magnesia, one day’s digestion not being sufficient for full extraction, it is evident that they do not exist in the soil as carbonates or hydric oxids only, as has been supposed, but also as silicates. A comparison of the results of the five and ten days’ digestion shows that the solvent action of the acid has substantially ceased at the end of five days, there being no further increase of the amount of dissolved matter.

332. Digestion Vessels.—Hilgard prescribes that the digestion of the sample of soil with acid be conducted in a small porcelain beaker covered with a watch-glass.^[215] Kedzie, however, prefers beakers of bohemian glass, and shows that hydrochloric acid attacks the porcelain with greater energy than the glass.^[216] Platinum would be the ideal material for the digestive vessels, but its great cost would exclude its general use. In most cases it will be found that the error introduced into the analysis by the use of porcelain or bohemian glass beakers is quite small and not likely to affect the quantitative estimation of soluble soil ingredients to any extent.

In this laboratory some comparative tests made by Mr. W. D. Bigelow have shown that vessels of hard glass of special manufacture are less soluble in hot hydrochloric acid of 1.115 specific gravity than porcelain, thus confirming the observation of Kedzie. Following are the data showing the weights of material dissolved in fifty hours:

In each case twenty-five cubic centimeters of the acid were used. The vessels all had approximately a capacity of 200 cubic centimeters.

333. Processes Employed—Hilgard’s Method.—The sample of soil sifted through a 0.5 millimeter mesh sieve and thoroughly air-dried, is conveniently preserved in weighing tubes. The actual content of hygroscopic and combined moisture may be previously made on a separate sample of soil.

In determining the amount of material to be employed for the general analysis regard must be had to the nature of the soil. This is necessary because of the impracticability of handling successfully such large precipitates of alumina as would result from the employment of as much as five grams in the case of calcareous clay soils; while in the case of very sandy soils even that quantity might require to be doubled in order to obtain weighable amounts of certain ingredients. For soils in which the insoluble portion ranges from sixty to eighty per cent, two and a half to three grams are about the right measure for general analysis, while for the phosphoric acid determination not less than three grams should be employed in any case. It has been alleged that larger quantities must be taken for analysis in order to secure average results. It is difficult to see why this should be true for soils and not for ores, in which the results affect directly the money value, while in the case of soils the interpretation of results allows much wider limits in the percentages. Correct sampling must be presupposed to make any analysis useful; but with modern balances and methods it is difficult to see why five grams should be employed instead of half that amount, which in some cases is still too much for convenient manipulation of certain precipitates.

The weighed quantity, usually of two to two and a half grams, is brought into a small porcelain beaker, covered with a watch-glass, treated with eight to ten times its bulk of hydrochloric acid of 1.115 specific gravity, and two or three drops of nitric acid, and digested for five days over the laboratory steam-bath. At the end of this time it is evaporated to dryness, first on the water-bath and then on the sand-bath. By this treatment all the silica set free is rendered insoluble.

334. Provisional Method of the Official Agricultural Chemists.—Place ten grams of the air-dried soil in a round bottom 150 to 200 cubic centimeter bohemian flask, add 100 cubic centimeters of pure hydrochloric acid of specific gravity 1.115, insert the stopper, wire it securely, place in a steam-bath, and digest for thirty-six hours at the temperature of boiling water. Pour the contents of the flask into a small beaker, wash with distilled water, add the washings to the contents of the beaker and filter through a washed filter. The residue is the amount insoluble in hydrochloric acid. Add a few drops of nitric acid to the filtrate, and evaporate to dryness on the water-bath; take up with hot water and a few drops of hydrochloric acid, and again evaporate to complete dryness. Take up as before, and filter into a liter-flask, washing with hot water. Cool and make up to the mark. This is solution A. The residue represents the silica originally dissolved. In comparing the two preceding methods it is found that the former; viz., digestion in flasks covered only with a watch-glass gives a larger quantity of dissolved matter in five days than the digestion under pressure does in thirty-six hours. In comparative tests in this laboratory made by Mr. W. D. Bigelow the respective quantities of soluble and insoluble matter obtained by the two methods in two soils are as follows:

335. The German Station Method.—The method recommended by the German Stations^[217] is greatly different from that described above, both in temperature and time of digestion. To one part of the soil are added two parts by volume of a twenty-five per cent hydrochloric acid solution, the quantity being increased to correspond to any excess of carbonates. The mixture is left for forty-eight hours with frequent shaking. As an alternate method, one part of soil is treated with two parts by volume of ten per cent hydrochloric acid, and heated on the water-bath, with frequent shaking, for three hours.

The soluble materials are determined in the filtrate by some of the methods usually employed.

336. The Gembloux Method.—The method of making the acid extract of the soil at the Gembloux Station does not differ greatly from some of those already described.

The quantity of air-dried material taken is such that it may weigh exactly 300 grams exclusive of the moisture which it contains. It is dried at 150° for at least six hours. The drying is necessary in order to obtain an extract in hydrochloric acid of exactly 1.18 specific gravity. The dry earth is placed in a flask of two or three liters capacity to which one liter of hydrochloric acid of 1.18 specific gravity is added, being careful to take precautions to prevent frothing if much carbonate be present. The acid is allowed to act for twenty-four hours, it being frequently shaken meanwhile. After settling it is decanted and filtered upon a double folded filter, the apex of which rests upon a small funnel covered with a plain filter of strong paper. Five hundred cubic centimeters of the filtrate are taken for the estimation, and in this filtrate are estimated the silica, phosphoric and sulfuric acids, potash, soda, iron, alumina, lime, and magnesia.

The filtrate is evaporated to dryness in a porcelain capsule, a few drops of nitric acid added and the liquid kept well stirred. The residue should be taken up with water, and if not perfectly bright a second and even a third evaporation with nitric acid should take place, until all the organic matter is destroyed, which will be indicated by the clear yellow or reddish-yellow color of the liquid, caused by the iron oxid. After the last evaporation the material is dried in a drying oven one hour at 110°.

337. Treatment with Cold Hydrochloric Acid.—According to the digestion method of Wolff^[218] the soil sample is treated with cold concentrated hydrochloric acid. The process is as follows:

Four hundred and fifty grams of the soil dried at 100° are placed in a glass flask and treated with 1,500 cubic centimeters of hydrochloric acid of 1.15 specific gravity, corresponding to thirty per cent of gaseous hydrochloric acid. For every five per cent of calcium carbonate which the soil may contain, an additional fifty cubic centimeters of hydrochloric acid are added. With frequent stirring, the soil is left in contact with the acid for forty-eight hours and then 1,000 cubic centimeters of liquid, as clear as possible, are poured off, which corresponds to 300 grams of the soil. After dilution with water it is filtered and the filtrate treated with a few drops of nitric acid and evaporated to dryness. After the separation of the silica the solution is again made up with water to 1,000 cubic centimeters.

Two hundred cubic centimeters of this solution, corresponding to sixty grams of the soil, are taken for the estimation of iron, alumina, lime, manganese, and magnesia.

Four hundred cubic centimeters of the solution, corresponding to 120 grams of the soil, are left for the estimation of sulfuric acid and alkalies. This method gives from five to six times less alkalies and a much smaller quantity of iron than the treatment with hot acid. In the use of hot acid, therefore, Wolff reduces the quantity of soil acted on to 150 grams.

338. Treatment with Nitric Acid.—For the purpose of estimating phosphoric acid Grandeau^[219] directs that the soil be extracted with nitric acid. For this purpose 100 grams of the air-dried fine earth are placed in a bohemian flask and treated cautiously with nitric acid in small quantities at a time. If the soil be calcareous in its nature it should be previously moistened with water, and the acid so added as to avoid undue effervescence, the flask being inclined during the operation. Sufficient acid is added to strongly saturate the sample and it is then digested on the sand-bath for two hours; or at least until the organic matters are destroyed, which will be indicated by the cessation of evolution of nitrous vapors. When the supernatant liquid has become clear it is decanted. The residue is washed with distilled water and separated on a filter, and washed until the wash-water is colorless. The decanted portion is united with the filtrate and the whole made up to a volume of one liter. The determinations are made in portions of 200 cubic centimeters each.

339. Digestion with Hydrofluoric and Sulfuric Acids.—When a complete disintegration of the siliceous substances in soils is desired as in analysis in bulk, the decomposition is easily accomplished by digestion with the above named acids in a platinum dish. The fine earth is saturated with a concentrated aqueous solution of hydrofluoric acid to which a few drops of sulfuric acid are added. It is then digested until nearly dry. If any undecomposed particles remain, the treatment is continued until complete decomposition is secured. The silica is thus all volatilized as hydrofluosilicic acid and the bases pre-existing in the soil are left as sulfates. This method of treatment is especially recommended when it is desired to estimate the whole quantity of any of the soil constituents with the exception of silica. The silica may, however, be determined in the distillate. Instead of using the solution of hydrofluoric acid, ammonium fluorid may be employed. In this process the sample of earth reduced to an impalpable powder by grinding in an agate mortar is mixed with four or five times its weight of the ammonium fluorid in a platinum dish and thoroughly moistened with sulfuric acid and allowed to stand at room temperature for several hours. It is then gently heated until all fumes of hydrofluosilicic acid have been driven off, but is not raised to a red heat. If any undecomposed particles remain, the above treatment is repeated.

DETERMINATION OF THE QUANTITY OF DISSOLVED MATTER.

340. Substances in Solution.—By treatment with solvents as indicated in the preceding paragraphs, greater or less quantities of the original constituents of soil are brought into solution. The total quantity of dissolved matters is determined by drying and weighing the insoluble residue and the percentages of soluble and insoluble matters should be noted; and each portion saved for further examination. In this country the common practice of soil analysis is to digest the sample with hydrochloric acid. The following paragraphs, therefore, will be devoted to the general methods of determining the matters dissolved by that treatment, leaving for later consideration the special methods of analysis. The fundamental principle on which the treatment with hydrochloric acid rests is based on the belief that such treatment practically extracts from the soil all those elements which are likely to become, immediately or in the near future, available for plant food.

341. Provisional Methods of the Official Agricultural Chemists.^[220]—(1) The Analytical Operations are conducted with solution A, paragraph 334.

(2) Ferric Oxid, Alumina, and Phosphoric Acid.—To 100 or 200 cubic centimeters, according to the probable amount of iron present, of the solution A, add ammonium hydroxid to alkaline reaction to precipitate ferric and aluminum oxids and phosphates. Expel the excess of ammonia by boiling, allow to settle, decant the clear solution through a filter; add to the flask fifty cubic centimeters of hot distilled water, boil, settle, and decant as before. After pouring off all the clear solution possible, dissolve the residue with a few drops of warm hydrochloric acid and add just enough ammonium hydroxid to precipitate the oxids. Wash by decantation with fifty cubic centimeters of distilled water, and then transfer all the precipitate to the filter and wash with hot distilled water till the filtrate becomes free from chlorids. Save the filtrate and washings which form solution B. Dry the filter and precipitate at 110°, transfer the precipitate to a tared platinum crucible, burn the filter and add the ash to the precipitate, heat the whole red hot, cool in a desiccator, and weigh. The increase of weight, minus the ash of filter and the phosphoric acid (found in a separate process), represents the weight of the ferric and aluminum oxids.

(3) Ferric Oxid.—Precipitate 100 cubic centimeters of solution A, as under (2), except that only one precipitation is made; wash with hot water; dissolve in dilute sulfuric acid; reduce with zinc and estimate as ferrous oxid by a standard solution of potassium permanganate.

To prepare the potassium permanganate solution, dissolve 3.156 grams of pure crystallized potassium permanganate in 1,000 cubic centimeters of distilled water, and preserve in a glass-stoppered bottle, shielded from the light. Standardize this solution with pure ferrous sulfate, ammonium ferrous sulfate or oxalic acid.

(4) Alumina.—The calculated weight of ferric oxid deducted from that of ferric oxid and alumina with corrections for filter ash and phosphoric acid, will give the weight of alumina in two grams of air-dried soil.

(5) Phosphoric Acid.—This may be estimated in the above iron solution, if the soil is sufficiently rich, by the molybdate method, given under fertilizers; or if the quantity of soil represented in the iron solution is not sufficient, a fresh portion of solution A may be taken, and the phosphoric acid determined directly by the molybdate method.

(6) Manganese.—Concentrate the filtrate and washings (solution B) to 200 cubic centimeters or less; add ammonium hydroxid to alkalinity; add bromin water and heat to boiling, keeping the beaker covered with a watch-glass; as the bromin escapes, the beaker is allowed to cool somewhat, ammonia and bromin water again added, and heated as before.

This process is continued until the manganese is completely precipitated, which requires from thirty to sixty minutes, and the solution filtered while still warm; the precipitate is washed, dried, ignited and weighed; estimate as manganese protosesquioxid.

(7) Lime.—If no manganese is precipitated, add to solution B, or the filtrate and washings from (6) twenty cubic centimeters of a strong solution of ammonium chlorid and forty cubic centimeters of saturated solution of ammonium oxalate to completely precipitate all the lime as oxalate and convert the magnesia into soluble magnesium oxalate. Heat to boiling and let stand for six hours till the calcium oxalate settles clear, decant the clear solution on a filter, pour fifty cubic centimeters of hot distilled water on the precipitate and again decant the clear solution on the filter, transfer the precipitate to the filter, and wash it free from all traces of oxalates and chlorids. Dry and ignite the precipitate over the blast-lamp until it ceases to lose weight, weigh and estimate as calcium oxid; carefully moisten with sulfuric acid, heat the inclined covered crucible gently to avoid loss, then intensely, and weigh as calcium sulfate.

(8) Magnesia.—Concentrate the filtrate and washings (from 7) to 200 cubic centimeters, place in a half-liter erlenmeyer, add thirty cubic centimeters of a saturated solution of sodium phosphate and twenty cubic centimeters of concentrated ammonium hydroxid, cork the flask, and shake violently at intervals of a few minutes till crystals form, then set the flask in a cool place for twelve hours. Filter the clear liquid through a tared gooch, transfer the precipitate to the filter, and wash with dilute ammonium hydroxid (1 : 3) till the filtrate is free from phosphates; dry and ignite the crucible, at first gently and then intensely, to form magnesium pyrophosphate. The increase of weight × 0.36024 = MgO. By using an erlenmeyer free from scratches and marks, and shaking violently instead of stirring with a glass rod, the danger is almost entirely avoided of crystals adhering to the sides of the vessel; but if crystals do adhere they are readily removed by a rubber-tipped glass rod.

(9) Sulfuric Acid.—Evaporate 200 cubic centimeters of solution A (1) nearly to dryness on a water-bath to expel the excess of acid; then add 100 cubic centimeters of distilled water, heat to boiling and add ten cubic centimeters of a solution of barium chlorid, and continue the boiling for five minutes. When the precipitate has settled, pour the clear liquid on a tared gooch, treat the precipitate with fifty cubic centimeters of boiling water, and transfer the precipitate to the filter and wash with boiling water till the filtrate is free from chlorids. Dry the filter and ignite strongly. The increase in weight is barium sulfate, which multiplied by 0.34331 = SO₃ in two grams of air-dried soil.

(10) Potash and Soda.—To another portion of 200 cubic centimeters of solution A, add barium chlorid in slight excess, and make alkaline with ammonia to precipitate sulfuric and phosphoric acids, ferric oxid, etc. Then precipitate the calcium and barium by ammonium oxalate. Evaporate the filtrate and washings to dryness, heat to a low red heat to decompose oxalates and expel ammonia salts, dissolve in twenty-five cubic centimeters of distilled water, filter and wash the precipitate; add to the filtrate and washings ten cubic centimeters of baryta water, and digest for an hour. Filter and wash the precipitate, add ammonium carbonate to the filtrate to complete precipitation of baryta, filter and wash this precipitate. Evaporate the filtrate and washings in a tared platinum dish, gently ignite the residue to expel ammonia salts, cool and weigh. The increase of weight represents the potassium and sodium chlorids in two grams of air-dried soil.

342. Hilgard’s Methods.^[221]—(1) Soluble Silica.— The acid filtrate obtained by the process given in paragraph 333 is employed for the following determinations. After the solution obtained has been evaporated to dryness to render silica insoluble, it is moistened with strong hydrochloric acid and two or three drops of nitric acid. The mass is warmed, and after allowing to stand for a few hours on a steam-bath is taken up with distilled water. After clearing, it is filtered from the insoluble residue, which is strongly ignited and weighed. If the filtrate should be turbid the insoluble residue which has gone through the filter can be recovered in the iron and alumina determination.

The insoluble residue is next boiled for fifteen or twenty minutes in a concentrated solution of sodium carbonate, to which a few drops of caustic lye should then be added to prevent reprecipitation of the dissolved silica. The solution must be filtered hot. The difference between the weight of the total residue and that of undissolved sand and mineral powder is recorded as soluble silica, being the aggregate of that set free by the acid treatment and that previously existing in the soil. The latter, however, rarely reaches five per cent.

(2) Destruction of Organic Matter.—The acid filtrate from the total insoluble residue is evaporated to a convenient bulk. In case the filtrate should indicate by its color, the presence of any organic matter, it should be oxidized by aqua regia, otherwise there will be difficulty in separating alumina.

(3) Precipitation of Iron and Alumina.—The filtrate thus prepared is now brought to boiling and treated sparingly with ammonia, whereby iron and alumina are precipitated. It is kept boiling until the excess of ammonia is driven off, and then filtered hot. (Filtrate A.) The previous addition of ammonium chlorid is usually unnecessary. If the boiling is continued too long, filtration becomes very difficult and a part of the precipitate may redissolve in washing. Filtration may be begun as soon as the nose fails to note the presence of free ammonia; test paper is too delicate. Failure to boil long enough involves the contamination of the iron-alumina precipitate with lime and manganese.

(4) Estimation of Iron and Alumina.—The iron and alumina precipitate with filter of (3) is dissolved in a mixture of about five cubic centimeters of hydrochloric acid and twenty cubic centimeters of water. Then filter and make up to 150 cubic centimeters. Take fifty cubic centimeters for the determination of iron and alumina together by precipitation with ammonia, after oxidizing the organic matter (filter) with aqua regia; also fifty cubic centimeters for iron alone; keep fifty cubic centimeters in reserve. Determine the iron by means of a standard solution of potassium permanganate after reduction; this latter is done by evaporating the fifty cubic centimeters almost to dryness with strong sulfuric acid, adding water and transferring the solution to a flask, and then reducing by means of pure metallic zinc in the usual way. The alumina is then determined by difference. This method of determining the two oxids in their intermixture is in several respects more satisfactory than the separation with alkaline lye, which, however, has served for most determinations made, until within the last ten years. It is, however, much more liable to miscarry in unpracticed hands than the other.

(5) Estimation of Lime.—The filtrate A from iron and alumina is acidified slightly with hydrochloric acid, and if too bulky is evaporated to about twenty-five cubic centimeters, unless the soil is a very calcareous one, and the lime is precipitated from it by neutralizing with ammonia and adding ammonium oxalate. The precipitation of the lime should be done in the hot solution, as the precipitate settles much more easily. It is allowed to stand for twelve hours, then filtered (filtrate B), washed with cold water, and dried. By ignition the lime precipitate is partially converted into the oxid. It is then heated with excess of powdered ammonium carbonate, moistened with water, and exposed to a gentle heat (50°–80°) until all the ammonia is expelled. It is then dried below red heat and weighed as calcium carbonate. When the amount of lime is at all considerable, the treatment with ammonium carbonate must be repeated till a constant weight is obtained.

(6) Estimation of Sulfuric Arid.—The filtrate B from the calcium oxalate is put into a bohemian flask, boiled down over the sand-bath, and the ammoniacal salts destroyed with aqua regia. From the flask it is removed to a small beaker and evaporated to dryness with excess of nitric acid. This process usually occupies four to five hours. The residue should be crystalline-granular; if white-opaque, ammonium nitrate remains and must be destroyed by hydrochloric acid. The dry residue is now moistened with nitric acid, and the floccules of silica usually present separated by filtration from the filtrate, which should not amount to more than ten or fifteen cubic centimeters; sulfur trioxid is then precipitated by treatment with a few drops of barium nitrate, both the solution and the reagent being heated to boiling. If the quantity of sulfuric acid is large it may be filtered after the lapse of four or five hours (filtrate C). If very small let it stand twelve hours. The precipitate is washed with boiling water, dried, ignited, and weighed. Care should be taken in adding the barium nitrate to use only the least possible excess, because in such a small concentrated acid solution the excess of barium nitrate may crystallize and will not readily dissolve in hot water. Care must also be taken not to leave in the beaker the large heavy crystals of barium sulfate, of which a few sometimes constitute the entire precipitate, rarely exceeding a few milligrams. Should the ignited precipitate show an alkaline reaction on moistening with water, it must be treated with a drop of hydrochloric acid, refiltered and weighed. The use of barium acetate involves unnecessary trouble in this determination.

(7) Estimation of Sodium and Potassium.—Filtrate C is now evaporated to dryness in a platinum dish; the residue is treated with an excess of crystallized oxalic acid, moistened with water, and exposed to gentle heat. It is then strongly ignited to change the oxalates to carbonates. This treatment with oxalic acid must be made in a vessel which can be kept well covered, otherwise there is danger of loss through spattering. As little water as possible should be used, as otherwise loss from evolution of carbon dioxid is difficult to avoid. Spatters on the cover should not be washed back into the basin until after the excess of oxalic acid has been volatilized. The ignited mass should have a slightly blackish tinge to prove the conversion of the nitrates into carbonates. White portions may be locally retreated with oxalic acid. The ignited mass is treated with a small amount of water, which dissolves the alkaline carbonates and leaves the magnesium carbonate, manganese protosesquioxid, and the excess of barium carbonate behind. The alkalies are separated by filtration into a small platinum dish (filtrate D), and the residue is well but sparingly washed with water on a small filter. When the filtrate exceeds ten cubic centimeters it may, on evaporation, show so much turbidity from dissolved earthy carbonates as to render refiltration on a small filter necessary, since otherwise the soda percentage will be found too large and magnesia too small. If, on dissolving the ignited mass, the solution should appear greenish from the formation of alkaline manganates, add a few drops of alcohol to reduce the manganese to insoluble dioxid. The residue of barium, magnesium, and manganese compounds is treated on the filter with hydrochloric acid, and the platinum dish is washed with warm nitric acid (not hydrochloric, for the platinum dish may be attacked by chlorin from the manganese oxid) dissolving any small traces of precipitate that may have been left behind.

The filtrate D, which should not be more than ten or fifteen cubic centimeters, containing the carbonates of the alkalies, is evaporated to dryness and gently fused, so as to render insoluble any magnesium carbonate that may have gone through; then redissolved and filtered into a small weighed platinum dish containing a few drops of dilute hydrochloric acid, to change the carbonates into chlorids; evaporated to dryness, exposed to a gradually rising temperature (below red heat), by which the chlorids are thoroughly dried and freed from moisture, so as to prevent the decrepitation that would otherwise occur on ignition. Then, holding the platinum basin firmly by forceps grasping the clean edge, pass it carefully over a very low bunsen flame, so as to cause, successively, every portion of the scaly or powdery residue to collapse, without fully fusing. There is thus no loss from volatilization, and no difficulty in obtaining an accurate, constant weight. The weighed chlorids are washed by means of a little water into a small beaker or porcelain dish, treated with a sufficient quantity of platinum chlorid, and evaporated to dryness over the water-bath. The dried residue is treated with a mixture of three parts absolute alcohol and one part ether, leaving the potassium platinochlorid undissolved. This is put on a filter, and washed with ether-alcohol. When dried, the precipitate and filter are put into a small platinum crucible and exposed to a heat sufficiently intense to reduce the platinum chlorid to metallic platinum and to volatilize the greater part of the potassium chlorid. This is easily accomplished in a small crucible, which is roughened by being constantly used for the same purpose (and no other), the spongy metal causing a ready evolution of the gases. The reduced platinum is now first washed in the crucible with hot acidulated water, then with pure water; then all moisture is driven off and it is weighed. From the weight of the platinum, is calculated the potassium chlorid and the oxid corresponding; the difference between the weights of the total alkaline chlorids and potassium chlorid gives the sodium chlorid, from which may be calculated the sodium oxid. When the heating of the platinum precipitate has not been sufficient in time or intensity, instead of being in a solid spongy mass of the color of the crucible itself, small black particles of metallic platinum will obstinately float on the surface of the water in the crucible, and it becomes difficult to wash without loss.

(8) Estimation of Manganese.—The solution containing the magnesium and manganese chlorids is freed from barium salts by hot precipitation with sulfuric acid, and the barium sulfate, after settling a few hours, is separated by filtration. The filtrate is neutralized with ammonia, any resulting small precipitate (of iron) is filtered, and the manganese precipitated with ammonium sulfid, let stand twelve hours and filtered (filtrate E); wash with cold water, dry, ignite, and weigh as manganese protosesquioxid, Mn₃O₄. If preferred the manganese may be precipitated with chlorin or bromin water as dioxid; but the process requires a rather longer time and may fail in inexpert hands more readily than the other.

(9) Estimation of Magnesium.—The filtrate E from the manganese is now freed from sulfur by acidulating with hydrochloric acid, evaporating, if necessary, and filtering. From the filtrate the magnesia is precipitated by adding an equal bulk of ammonia water and then sodium phosphate. After standing at least twenty-four hours, the magnesium salt may be filtered, washed with ammoniacal water, dried, ignited, and weighed as magnesium pyrophosphate.

343. Examination of Acid Extract by the Methods of Petermann.—Estimation of the Silica.—The Gembloux method of estimating silica consists in taking up the dry extract obtained from the treatment of the earth, in the manner described in paragraph 336, with water and a few drops of hydrochloric acid, heating for a short time on a sand-bath to facilitate the solution, and filtering, washing, drying, igniting, and weighing the residue obtained as silica.

Estimation of the Sulfuric Acid.—The method employed consists in heating the filtrate obtained in the estimation of silica for half an hour with a few drops of nitric acid and making the volume up to 500 cubic centimeters. One hundred cubic centimeters of this are precipitated with barium chlorid, diluted to double its volume, heated for some time, the precipitate of barium sulfate collected and weighed, and the quantity of sulfuric acid calculated therefrom.

Potash and Soda.—Potash and soda are estimated at the Gembloux Station by heating the filtrate obtained in the estimation of the sulfuric acid and precipitating the excess of barium in the hot solution after the addition of ammonia by ammonium oxalate and carbonate. The whole is allowed to digest for six hours at a gentle heat and then allowed to remain at rest for twenty-four hours, filtered, washed, and the filtrate evaporated to dryness in a large platinum dish and the ammoniacal salts driven off at a low temperature. At the end, the temperature is carried a little higher until it reaches low redness. The residue is taken up by distilled water, filtered into a weighed platinum dish, a few drops of hydrochloric acid added, evaporated, dried, heated with great care and the sodium and potassium chlorids obtained weighed together. The respective quantities of potash and soda in the earth are estimated in the usual way by precipitating the potash with platinum chlorid.

Estimation of the Iron and Aluminum Oxids.—The iron and aluminum oxids are estimated by taking twenty-five cubic centimeters of the primitive solution obtained with hydrochloric acid and adding ammonium carbonate almost to complete neutralization, that is to say until the precipitate formed is just redissolved in the feeble excess of hydrochloric acid which remains. Dilute with distilled water and precipitate with a little excess of ammonium acetate, and boil for a moment; after boiling, the basic iron and aluminum acetate and the small quantity of iron and aluminum phosphate present are easily deposited, and the supernatant liquid should be completely limpid and colorless. Wash the precipitate by decantation, boiling each time, filter, wash the filter with boiling water to which a little ammonium acetate has been added, dry, ignite, and weigh. The material obtained consists of ferric oxid, aluminum oxid, and iron and aluminum phosphates. Deduct from the whole, the phosphoric acid determined in another portion. The residue will be the sum of the iron and aluminum oxids.

Estimation of the Lime.—The filtrate from the portion used for the estimation of the iron and alumina is treated with ammonium oxalate. The mixture is kept at a low temperature for at least twelve hours, after which it is filtered, washed with hot water, dried, and ignited over a blast-lamp to constant weight and weighed as calcium oxid.

Estimation of the Magnesia.—For the estimation of the magnesia the filtrate obtained in the estimation of lime is evaporated to dryness in a platinum dish, the ammoniacal salts driven off, the residue taken up with water slightly acidified with hydrochloric acid, filtered, the filtrate saturated with ammonia and heated some time to the boiling point to precipitate any traces of iron and alumina which may have remained in solution. Filter, wash, allow to cool and precipitate the magnesia by the addition of sodium phosphate. It is then allowed to stand for twelve hours, collected on a filter, ignited, and weighed as pyrophosphate, and the quantity of magnesia calculated from the weight of salt obtained.

Estimation of the Phosphoric Acid.—The phosphoric acid is estimated by taking 100 cubic centimeters of the original solution obtained by the treatment of the soil with hydrochloric acid and evaporating it to dryness on the water-bath. The residue is taken up with water to which a few drops of nitric acid have been added and filtered. The total phosphoric acid is then obtained by precipitation with ammonium molybdate in the usual way.

344. Analysis of the Insoluble Residue.—The insoluble residue left after digestion with hydrochloric acid is not without interest from an agricultural and analytical point of view. While it is true that the plant food, therein contained, is not immediately available, yet it must not be forgotten that the method of the chemist may not fix a limit to nature’s method of collecting nutriment for plants. In however refractory a state they may exist, it is possible that all nutritive elements may eventually become available for assimilation. For the completion of an estimate of the total nutritive power of a soil, therefore a further examination of the insoluble residue should be made. The methods of securing this are essentially those of making a bulk analysis of the soil.

The principle of the method depends on the reduction of the sample to an impalpable powder and the subsequent decomposition of the insoluble portions by treatment with hydrofluoric and sulfuric acids or by fusion with the alkalies.

345. Method of Wolff for Treating Residue Insoluble in Hot Acid.—The well-washed residue is dried with the filter, then separated therefrom, the filter burned and the ash weighed with the whole of the residue. About eight grams of the residue are ignited and serve for the estimation of the insoluble mineral matter. Another portion of ten grams of the dried, but not ignited, residue is boiled with a concentrated solution of sodium carbonate with the addition of caustic soda, and the quantity of dissolved silicic acid estimated. A third portion of about fifteen grams is treated with about five times its weight of pure concentrated sulfuric acid, and is evaporated until the mass has taken the form of a dry powder. After moistening with concentrated hydrochloric acid the mass is boiled with water, filtered, and the filtrate examined according to the ordinary methods for silicic acid, alumina, iron, lime, magnesia, and alkalies. The residue after treatment with concentrated sulfuric acid is dried, but not ignited, and boiled with a concentrated solution of sodium carbonate with the addition of a little caustic soda, filtered, heated, and the silicic acid separated from the solution. After thorough washing, the residue, after ignition, is weighed and represents the material insoluble in concentrated hydrochloric and sulfuric acids. The silicic acid found as before, together with the small quantity dissolved in the hydrochloric acid extract, gives, in connection with the alumina contained in the sulfuric acid extract, approximately the quantity of pure water-free clay contained in the soil.

In six samples of soils of very different compositions which were examined by the above process, it was found that the clay had the following mean composition: Silicic acid, 55.1 to 61.5 per cent, alumina, 38.6 to 44.9 per cent; as a mean 58.05 per cent silicic acid and 41.95 per cent alumina.

Finally, four or five grams of the residue, after treatment with sulfuric acid and sodium carbonate, are rubbed up in an agate mortar and completely separated into silt by water. The silt mass is dried, lightly ignited, and three grams of it spread in a flat platinum dish moistened with sulfuric acid, and subjected to the action of hydrofluoric acid in a lead oven at 60°, until a complete decomposition of the material is accomplished. In the solution all the different bases can be determined.

346. Method of the Belgian Chemists.—The method employed by Petermann^[222] at the Gembloux Station in the examination of the part of the soil insoluble in hydrochloric acid consists in washing the insoluble portion by decantation with distilled water until all acid reaction is removed. Place the contents of the flask and of the filter in a porcelain dish and dry. After a careful mixing of the mass take out about fifty grams and wash upon the filter until all reaction for chlorin has disappeared, dry, detach the mass from the filter, and incinerate. Place in a platinum crucible two grams of the ground and ignited residue and mix it, using a platinum stirring rod, with twelve grams of ammonium fluorid; heat slightly over a bunsen burner in a muffle with a good draught and regulate the flame in such a way that the operation shall continue for about one hour. After complete decomposition add about two cubic centimeters of sulfuric acid in such a way as to moisten completely the residue, drive off the sulfuric acid carefully at a low red heat and take up the residue with water slightly acidulated with hydrochloric acid and wash the whole into a flask of 500 cubic centimeters capacity. Oxidize by heating for an hour with nitric acid, make up to the mark and filter. The percentages of potash, soda, lime, magnesia, and the silicates are determined exactly as in the hydrochloric acid extract.

347. Bulk Analysis.—It is frequently desirable to determine the total composition of a soil sample as well as the nature of that part of it soluble in any of the solvents usually employed. The latest methods for this purpose have been well studied by Packard^[223] who finds that the variations which occur between duplicates are probably due to the small quantities of material taken for analysis, it being difficult to obtain average samples of a material which is not very finely powdered when small quantities are taken. Moreover, as it is likely to become of importance to know whether the proportions of lime and magnesia vary by as much as one-tenth per cent, and such small variations are within the limits of error of an analysis, and as the total proportion of lime and magnesia in highly siliceous soils, probably does not exceed one-tenth per cent, it is deemed best to take a large quantity of soil for the bulk analysis in each case. The amount adopted for the highly siliceous soils containing much quartz is ten grams. This quantity, taken after quartering down the entire sample, is ground to an impalpable powder and used for the determination of the lime, magnesia, and alkalies, the silica, iron oxid, alumina, and loss on ignition, being determined in one gram samples. The ten grams are decomposed by hydrofluoric and sulfuric acids in a large platinum dish, the solution evaporated, at first on the water-bath until all water is removed and then at a higher temperature until all the free sulfuric acid is driven off, when the residue is heated in a muffle at a low red heat for several hours. At this temperature the sulfuric acid combined with the iron oxid and alumina is driven off, leaving the remaining sulfates unchanged and the iron oxid and alumina are in the form of a powder of no great volume which is easily and quickly washed. This operation is usually successful at first but in some cases the decomposition is not complete as is shown by the appearance of a precipitate on adding ammonia to the filtrate from the aluminum and iron oxids. In such cases the precipitate is dissolved in hydrochloric acid, reprecipitated by ammonia and removed by filtration. In the filtrate from the thoroughly washed aluminum and iron oxids, lime is precipitated as oxalate and separated by filtration; the filtrate is evaporated to dryness and the ammonia salts driven off by heat; the magnesia in the unfiltered watery extract of this residue is precipitated by baryta water, which also removes the sulfuric acid with which the bases had been combined. In the filtrate from this precipitate baryta is precipitated by ammonium carbonate and removed by filtration, leaving the alkalies to be determined in the usual way after conversion into chlorids. The mixed precipitate of magnesia and barium sulfate is treated with hydrochloric acid, filtered, the baryta present removed as sulfate, and the magnesia precipitated in the filtrate from the latter as phosphate. The advantages of this method are that the large quantity of material employed gives some assurance that an average sample has been operated on, and all the bases present in small proportions are estimated in the same sample. The objection to it is the time consumed both in grinding the samples and in determining all the bases in one solution. As a small quantity of material is generally used for determining the silica, iron oxid, alumina, and loss by ignition, and a larger quantity for the remaining bases, slight differences in the unground samples are unavoidable, especially when the quartz grains are somewhat large, it being practically impossible to take two small samples of such a soil which would have the same number of quartz grains. Consequently tedious grinding of large quantities of the soils for the bulk analysis is necessary. This objection does not apply to the official analysis or assay of soils in which considerable quantities are extracted by acid and the solution analyzed, and silica is not determined. In any case, it may be said, when it becomes an object to know whether a soil contains a total of 0.1 or 0.2 per cent of lime or magnesia, of 0.7 or 0.5 per cent of potash, one analysis even of the large quantity of ten grams would be insufficient to decide the point, and at least the mean of two determinations should be taken.

SPECIAL METHODS OF DETERMINATION OF SOIL CONSTITUENTS.

348. Preliminary Considerations.—In the foregoing paragraphs the general outline of the chemical methods of soil examination have been given. There are often occasions, however, which demand a special study of some particular soil constituent. It has been thought proper, therefore, to add here some of the best approved methods of special determinations which have been approved in this and other countries.

In the main, the final determination of any particular element of the soil, and its previous separation from accompanying elements, are based on the general processes already given. The variations in many instances, however, seem to require special mention.

349. Condition of Potash in Soils.—Potash exists in the soil in very different states. That part of it which is combined with the humus material, or with the hydrated silicates, is easily set free from its combinations and is to be regarded as the more assimilable portion.

The potash in the soil is found chiefly in combination with silicates, and particularly with the hydrated aluminum silicates, forming clay. As the particles with which it is combined are found in a state of greater or less fineness, the potash itself is set free under the influences of the agents which are active in the soil, with greater or less rapidity, passing into a form in which it can be utilized by plants. In silicates which are very finely divided, such as clay, the potash becomes active in a relatively short time, while in the débris of rocks in a less advanced state of decomposition it may rest for an indefinite period in an inert state. The estimation of the potash which is assimilable in the clay is quite as important for agricultural purposes as to determine that which may be present in the soil in firmer combination. Treating the sample of soil with water does not furnish any useful information in regard to the potash which it contains. Indeed, the absorbing properties of the soil tend to prevent the elimination of the potash in this way, even when it is found in the soluble state. It is therefore, necessary to employ an acid to set the potash free, but variable results are obtained, according to the employment of acids of greater or less concentration and for longer or shorter periods of contact.

350. Estimation of the Potash Soluble in Concentrated Acids.—In the method of the French agricultural chemists^[224] twenty grams of the earth are placed in a dish with a flat bottom, eleven centimeters in diameter, and rubbed up with twenty to thirty cubic centimeters of water. There is added carefully, and in small quantities, some nitric acid of 36° Baumé until all effervescence has ceased, the mass meanwhile being thoroughly stirred. When the carbonates have been decomposed, which can be told by the cessation of the effervescence, twenty cubic centimeters more of the same acid are added. The dish is heated on the sand-bath for five hours, regulating the heating in such a way that there still remains some acid at the end of the operation and the mass is not thoroughly dry. The acid mass is then taken up with hot water, filtered, and washed with hot water until the amount of filtrate is about 300 cubic centimeters. The filtrate should be received in a flask of about one liter capacity. The filtrate will contain the dissolved potash, soda, magnesia, lime, iron and aluminum oxids, and traces of sulfuric and phosphoric acids. For the elimination of the other substances, with the exception of potash, soda and magnesia, a few drops of barium nitrate are added, afterwards sufficient ammonia to render the solution alkaline, and finally an excess of ammonium carbonate in powder added in small portions. These materials are added successively and the whole is left to stand for twenty-four hours. By this operation the sulfuric acid is separated in the form of barium sulfate; the iron and aluminum oxids are precipitated, carrying down with them the phosphoric acid, and the lime is thrown down in the form of carbonate. The mass is now filtered and washed several times with hot water. The filtrate contains in addition to potash, soda, magnesia, and the ammoniacal salts which have been introduced. The ammonium salts are destroyed by adding aqua regia and evaporating the liquid to a very small volume, as described in the method for the estimation of magnesia. The mass is now evaporated in a porcelain dish with a flat bottom, of about seven centimeters diameter, and an excess of perchloric acid added. The evaporation is carried to dryness on a sand-bath, and the heating prolonged until the last white fumes of perchloric acid are disengaged. The mass is now left to cool. There are then added five cubic centimeters of alcohol, of 90° strength. The mass is triturated by a stirring rod, the extremity of which is flattened, in such a manner as to reduce it all to an impalpable powder. It is then left to settle and the supernatant liquid is decanted upon a small filter. The treatment with alcohol of the kind, quantity, and strength described, is continued four or five times. Afterwards, as there may still remain a trace of the sodium and magnesium perchlorates in the interior of the crystals of potassium perchlorate, there are added to the capsule in which all of the alkaline residue has been collected, two or three cubic centimeters of water, and it is evaporated again to dryness and taken up twice with small quantities of alcohol. There are thus removed the traces of sodium and magnesium perchlorates. By means of a jet of boiling water the stirring rod and the filter, which contains the small quantities of potassium perchlorate, are washed, and the liquid passing through is received in the capsule which contains the larger part of the salt. It is then evaporated to dryness and weighed.

When there is very little magnesia present, as is generally the case, the estimation of the potash is made without any difficulty by the process just mentioned, but when the proportion of magnesia is high it is found useful to separate it before the transformation into perchlorates. The magnesia is separated by carbonating the residue as indicated in the method for the estimation of magnesia, by treatment with oxalic acid and ignition. By extracting the carbonates formed with very small quantities of water, and filtering, the alkalies are obtained free from magnesia.

It is advisable to test the purity of the potassium perchlorate formed which sometimes contains a little silica. For this purpose it is dissolved in boiling water, and any residue which remains is weighed, and that weight deducted from the total weight of perchlorate. By multiplying the weight of potassium perchlorate found by the coefficient 0.339, the quantity of potash contained in the twenty grams of earth submitted to analysis is obtained.

Estimation of the Potash Soluble in Cold, Dilute Acids.—(Method of Schloesing.) Introduce 100 grams of the soil into a one or one and a half liter flask with 600 to 800 cubic centimeters of water. A little nitric acid, of 30° Baumé, is added until the carbonate is decomposed and a slight acid reaction is obtained. Afterwards five cubic centimeters of the same acid are added and it is left to digest for six hours, shaking every fifteen minutes. Instead of taking the whole of the wash-water for the examination, it is better to extract only a portion of it and so dispense with washing. This process is conducted in the following manner: