antimonic acid, and a fused mass is produced which appears gray from the minutely divided reduced metals in it. If the tube is now dipped whilst still red-hot into a test-tube filled with cold water, the part containing the fused mass will crack off, and the broken fragments sink to the bottom of the water. Upon application of heat, the fused mass will readily dissolve, leaving the metals behind, which may now by repeated boiling with water and decantation be freed from the last traces of other matters still adhering to them. Heat the residuary metallic mass with hydrochloric acid just to the boiling point, when the tin or a portion of it will dissolve to protochloride, with evolution of hydrogen gas; by means of chloride of mercury or a mixture of ferricyanide of potassium and sesquichloride of iron you may now easily, and with positive certainty, detect the protochloride of tin in the solution, even though only a minute trace of the metal be present. Upon heating the residue remaining in the tube repeatedly with hydrochloric acid, the whole, or nearly the whole of the tin present is dissolved out, and the black-colored antimony alone is left undissolved. Upon heating this residue with hydrochloric acid, with addition of one or two drops of nitric acid, it dissolves, and if you now add to the solution hydrosulphuric acid, an orange-colored precipitate of tersulphide of antimony is obtained. If only slight traces of antimony are present, the color of this precipitate is not very distinct, owing to imperfect separation from the tin. Let, therefore, the precipitate of the impure tersulphide of antimony settle, decant the clear fluid, dissolve the precipitate in a little boiling hydrochloric acid, concentrate the solution somewhat, and then test in the lid of a platinum crucible with zinc, which will at once remove all doubt, showing even the minutest trace of antimony.

2. Instead of incinerating the filter with the binoxide of tin and antimonate of soda, you may also dissolve the precipitate on it in a little hot hydrochloric acid, wash the filter with water, concentrate the fluid somewhat, and test with zinc in a small platinum dish, when the antimony will at once reveal its presence by staining the platinum brownish-black; to detect the tin, wait until the little slip of zinc is almost entirely dissolved, then decant the fluid, heat the residue with some hydrochloric acid, and test the solution for protochloride of tin as in 1.

3. If the quantity of binoxide of tin and antimonate of soda is somewhat larger, the two bodies may be separated also by boiling with concentrated solution of soda, which dissolves the binoxide of tin, leaving the antimonate of soda unaffected. It is advisable to add, after the process of boiling, some spirit of wine, say about by volume of the entire fluid, to guard against the antimonate of soda being dissolved; for the same reason the precipitate must be washed with equal volumes of water and spirit of wine. If the alkaline solution is now evaporated to drive off the spirit of wine, hydrochloric acid then added, and lastly, hydrosulphuric acid, the tin is precipitated as sulphide; the antimony may in like manner be precipitated from the solution of the antimonate of soda in hydrochloric acid.

4. If the mixed sulphides are treated with fuming hydrochloric acid, the sulphide of antimony and sulphide of tin dissolve, whilst the sulphide of arsenic is left nearly altogether undissolved. If the residuary sulphide of arsenic is treated with ammonia, and the solution obtained evaporated, after adding to it a very little carbonate of soda, an arsenical mirror may readily be produced from the residue with cyanide of potassium and

Or, if

carbonate of soda in a stream of carbonic acid gas. The solution, which contains the tin and antimony, may be treated as directed in 2. there is a considerable excess of antimony, the solution may be mixed with sesquicarbonate of ammonia in excess, and boiled. A considerable proportion of the antimony present dissolves in this process, whilst binoxide of tin mixed with a small quantity of teroxide of antimony remains undissolved. The presence of the tin may now be the more readily proved by the method given in 1 (Bloxam).

5. If sulphide of antimony, sulphide of tin, and sulphide of arsenic are dissolved in sulphide of potassium, a large excess of a concentrated solution of sulphurous acid added, the mixture digested for some time in the water-bath, then boiled until all sulphurous acid is expelled, and filtered, the filtrate contains all the arsenic as arsenious acid (which may be precipitated from it by hydrosulphuric acid), whilst tersulphide of antimony and bisulphide of tin are left behind undissolved (Bunsen). After dissolving the residue in hydrochloric acid, the antimony and tin may be detected as directed in 2.

6. In the analysis of alloys, binoxide of tin and teroxide of antimony are often obtained together as a residue insoluble in nitric acid. If the quantities are considerable, the teroxide of antimony may be extracted with tartaric acid, and the solution tested for it with hydrosulphuric acid; the binoxide of tin may be reduced with cyanide of potassium. For minute quantities I recommend the method described in 1. An absolute separation of the two oxides may be effected by fusing them with hydrate of soda in a silver crucible, treating the mass with water, and adding one-third (by volume) of spirit of wine. The binoxide of tin is by this means obtained in solution as a compound of binoxide of tin and soda, whilst the antimonate of soda is left undissolved (H. Rose).

7. For the most accurate way of separating antimony and arsenic, and distinguishing between the two metals, viz., by treating with hydrosulphuric acid the mirror produced by Marsh's method, and separating the resulting sulphides by means of hydrochloric acid gas, I refer to § 131, 10. Antimony and arsenic may, however, when mixed together in form of hydrides, be separated also in the following way: Conduct the gases mixed with an excess of hydrogen, first through a tube containing glass splinters moistened with solution of acetate of lead, to retain the hydrochloric and hydrosulphuric gas, then in a slow stream into a solution of nitrate of silver. All the antimony in the gas falls down as black antimonide of silver, whilst the arsenic remains in solution as arsenious acid, and may, after precipitation of the excess of silver by hydrochloric acid, be readily detected in the fluid.

Protoxide and binoxide of tin may be detected and identified in presence of each other, by testing one portion of the solution containing both oxides for the protoxide with chloride of mercury, terchloride of gold or a mixture of ferricyanide of potassium and sesquichloride of iron, and another portion for the binoxide, by pouring it into a concentrated solution of soda.

Teroxide of antimony in presence of antimonic acid may be identified by the reaction described in § 130, 9. Antimonic acid in presence of teroxide of antimony, by heating the teroxide suspected to contain an admixture of the acid, but without any other admixture, with hydrochloric acid free from chlorine, and iodide of potassium free from iodate of potassa. If antimonic acid is present, iodine will separate, which, in

the case of larger quantities of acid, may be known from the brown color exhibited by the fluid, in the case of only slight traces of acid, after addition of a few drops of bisulphide of carbon (see § 154, 10). (Bunsen.)

Arsenious acid and arsenic acid in the same solution may be distinguished by means of nitrate of silver. If the precipitate contains little arsenate and much arsenite of silver, it is necessary, in order to identify the former, to add cautiously and drop by drop most highly dilute nitric acid, which dissolves the yellow arsenite of silver first.

A still safer way to detect small quantities of arsenic acid in presence of arsenious acid, is to precipitate the solution which contains the two acids, with a mixture of sulphate of magnesia, chloride of ammonium, and ammonia. The precipitate formed may be further examined by dissolving it in a very small quantity of nitric acid, mixing the solution with nitrite of silver, and then very cautiously adding dilute ammonia, which will lead to the formation of a precipitate of brownish-red arsenate of silver, if arsenic acid is present. Arsenious acid in presence of arsenic acid may also be identified by the reduction of oxide of copper effected by its agency. To distinguish between the ter- and the pentasulphide of arsenic, boil the potassia solution of the sulphide of arsenic under examination with hydrate of teroxide of bismuth, filter off from the tersulphide of bismuth formed, and test the filtrate for arsenious and arsenic acids. If silver is to be used as the reagent to distinguish between the two acids in the filtrate, the sulphide of arsenic may also be dissolved in ammonia, the solution mixed with nitrate of silver, the fluid filtered off from the sulphide of silver formed, and the filtrate cautiously tested with dilute nitric acid, to see whether a yellow or a brown precipitate or a mixture of both is produced.

$134.

SUPPLEMENT TO THE SIXTH GROUP.

MOLYBDIC ACID (Mo O1).

Molybdenum is silvery-white, the protoxide of the metal (Mo O) is black, the binoxide (Mo O,) dark brown. The metal and the two oxides, when heated in the air, oxidize to molybdic acid (Mo 02). Molybdic acid is a white, porous mass, which in water separates as fine scales; it fuses at a red-heat; in close vessels it volatilizes only at a very high temperature, in the air easily at a red-heat, subliming to transparent lamine and needles. The non-ignited acid dissolves in acids. The solutions are colorless; in contact with zinc or tin they first turn blue, then green, and ultimately black, with separation of protoxide of molybdenum; when digested with copper, they acquire a red tint, in consequence of ensuing reduction of the acid to binoxide. Ferrocyanide of potassium produces a reddish-brown precipitate, infusion of galls a green precipitate. Hydrosulphuric acid, when added in small proportion, imparts a blue tint to solutions of molybdic acid; when added in larger proportion, it produces a brownish-black precipitate; the fluid over the latter at first appears green, but after standing some time, and upon application of heat, it deposits an additional portion of brownishblack tersulphide of molybdenum (Mo S,). The precipitated tersulphide

of molybdenum dissolves in sulphides of the alkali metals; acids reprecipitate from the sulphur salts the sulphur acid (Mo S). When roasted at a red-heat in the air or heated with nitric acid, sulphide of molybdenum is converted into molybdic acid.

Molybdic acid dissolves readily in solutions of pure alkalies and carbonates of the alkalies; from rather concentrated solutions, sulphuric, nitric, and hydrochloric acids throw down molybdic acid, which redissolves upon further addition of the precipitant. The solutions of

molybdates of the alkalies are colored yellow by hydrosulphuric acid, and give afterwards, upon addition of acids, a brownish-black precipitate. For the deportment of molybdic acid with phosphoric acid and ammonia, see § 143, 11.

B.-DEPORTMENT OF THE ACIDS AND THEIR RADICALS WITH

REAGENTS.
§ 135.

The reagents which serve for the detection of the acids are divided, like those used for the detection of the bases, into GENERAL REAGENTS, i. e. such as indicate the GROUP to which the acid under examination belongs; and SPECIAL REAGENTS, i.e., such as serve to effect the detection and identification of the INDIVIDUAL ACIDS. The groups into which we classify the various acids can scarcely be defined and limited with the same degree of precision as those into which the bases are divided.

The two principal groups into which acids are divided are those of INORGANIC and ORGANIC ACIDS. We base this division upon those characteristics by which, irrespectively of theoretical considerations, the ends of analysis are most easily attained. We select therefore here, as the characteristic mark to guide us in the classification into organic and inorganic acids, the deportment which the various acids manifest at a high temperature, and call organic those acids of which the salts-(particularly those which have an alkali or an alkaline earth for base)—are decomposed upon ignition, the decomposition being attended with separation of carbon.

By selecting this deportment at a high temperature as the distinctive characteristic of organic acids, we are enabled to determine at once by a most simple preliminary experiment the class to which an acid belongs. The salts of organic acids with alkalies or alkaline earths are converted into carbonates when heated to redness.

Before proceeding to the special study of the several acids considered in this work, I give here, the same as I have done with the bases, a general view of the whole of them classified in groups.

§ 136.

CLASSIFICATION OF ACIDS IN GROUPS.

I. INORGANIC ACIDS.

FIRST GROUP:

Division a. Arsenious acid, arsenic acid, chromic acid (selenious acid, sulphurous and hyposulphurous acids, iodic acid).

Division b. Sulphuric acid (hydrofluosilicic acid).

Division c. Phosphoric acid, boracic acid, oxalic acid, hydrofluoric

acid.

Division d. Carbonic acid, silicic acid.

SECOND GROUP:

Chlorine and hydrochloric acid, bromine and hydrobromic acid, iodine and hydriodic acid, cyanogen and hydrocyanic acid, together with hydroferro- and hydroferricyanic acids, sulphur and hydrosulphuric acid (nitrous acid and hypochlorous acid).

THIRD GROUP:

Nitric acid, chloric acid.

II. ORGANIC ACIDS.

FIRST GROUP:

Oxalic acid, tartaric acid, citric acid, malic acid (racemic acid).

SECOND GROUP:

Succinic acid, benzoic acid.

THIRD GROUP:

Acetic acid, formic acid.

I. INORGANIC ACIDS.

§ 137.

First Group.

ACIDS WHICH ARE PRECIPITATED FROM NEUTRAL SOLUTIONS BY CHLORIDE OF BARIUM.

This group is again subdivided into four divisions, viz. :

1. Acids which are decomposed in acid solution by hydrosulphuric acid, and to which attention has therefore been directed already in the testing for bases, viz., ARSENIOUS ACID, ARSENIC ACID, and CHROMIC (In a supplement, page 129, I give selenious acid, sulphurous' acid, and hyposulphurous acid, the latter because it is decomposed and detected by the mere addition of hydrochloric acid to the solution of one of its salts; and also iodic acid.)

ACID.

2. Acids which are not decomposed in acid solution by hydrosulphuric acid, and the baryta compounds of which are insoluble in hydrochloric acid. Of the acids claiming our attention here, SULPHURIC ACID alone belongs to this class. (In a supplement, page 132, I give hydrofluosilic acid.)

3. Acids which are not decomposed in an acid solution by hydrosulphuric acid, and the baryta compounds of which dissolve in hydrochloric acid, apparently WITHOUT DECOMPOSITION, inasmuch as the acids cannot be completely separated from the hydrochloric acid solution by heating or evaporation; these are PHOSPHORIC ACID, BORACIC ACID, OXALIC ACID, and HYDROFLUORIC ACID. (Oxalic acid belongs more properly to the organic group. We consider it, however, here with the acids of the inorganic class, as the property of its salts to be decomposed upon ignition without actual carbonization may lead to its being overlooked as an organic acid.)

4. Acids which are not decomposed in acid solution by hydrosulphuric acid, and the baryta salts of which are soluble in hydrochloric acid, WITH DECOMPOSITION (separation of the acid): CARBONIC ACID, SILICIC

ACID.