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 a 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 0). Molybdenum is silvery-white, the protoxide of the metal (Mo O) is black, the binoxide (Mo O2) dark brown. The metal and the two oxides, when heated in the air, oxidize to molybdic acid (Mo 0 ̧). 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. 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) — decomposed upon ignition, the decomposition being attended with separation of carbon. -are 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. First Division of the First Group of the Inorganic Acids. § 138. a. ARSENIOUS ACID and ARSENIC ACID are, as we have seen above, decomposed by hydrosulphuric acid, and are precipitated by that reagent respectively as ter- and pentasulphide of arsenic. As this would lead to confounding them rather with the metallic oxides thau with other acids, it has been deemed more judicious to class these two acids with the oxides of the sixth group. (See § 131 and § 132.) b. CHROMIC ACID (Cr O). 1. Chromic acid presents the appearance of a scarlet-red crystalline mass, or of distinct acicular crystals. Upon ignition it is resolved into sesquioxide of chromium and oxygen. It deliquesces rapidly upon exposure to the air. It dissolves in water, imparting to the fluid a deep reddish-brown tint, which remains still visible in very dilute solutions. 2. The chromates are all red or yellow, and for the most part insoluble in water. Part of them are decomposed upon ignition; those with alkaline bases are fixed, and soluble in water; the solutions of the neutral alkaline chromates are yellow, those of the alkaline bichromates are red. These tints are still visible in highly dilute solutions. The yellow color of the solution of a neutral salt changes to red on the addition of a mineral acid, owing to the formation of an acid chromate. 3. Hydrosulphuric acid reduces chromic acid readily when present in the solution in the free state, more difficultly when present in form of a chromate; sesquioxide of chromium, water, and sulphuric acid are formed in this process, and sulphur precipitates. Heat promotes this decomposition. If no free acid is present, a greenish-gray precipitate is produced, consisting of a mixture of hydrated sesquioxide of chromium and sulphur. But if free acid is present, the precipitate is far less considerable, and consists of pure sulphur. In the latter case, the salt of sesquioxide of chromium formed imparts a green tint to the fluid, which may lead to the erroneous impression that the precipitate itself is green. 4. Chromic acid may also be reduced to sesquioxide of chromium by means of many other substances, and more particularly by sulphurous acid, or by heating with hydrochloric acid, especially upon the addition of alcohol (in which case chloride of ethyle and aldehyde are evolved); also by metallic zinc, or by heating with tartaric acid, oxalic acid, &c. All these reactions are clearly characterized by the change of the red or yellow color of the solution to the green tint of the salt of sesquioxide of chromium. 5. Chloride of barium produces a yellowish-white precipitate of CHROMATE OF BARYTA (Ba O, Cr O,), which is soluble in dilute hydrochloric acid and nitric acid. 6. Nitrate of silver produces a dark purple-red precipitate of CHROMATE OF SILVER (Ag O, Cr O,), which is soluble in nitric acid and in ammonia; in acid solutions it produces a precipitate of BICHROMATE OF SILVER (Ag O, 2 Cr 01). 7. Acetate of lead produces a yellow precipitate of CHROMATE OF LEAD (Pb O, Cr O1), which is soluble in potassa, but only difficultly soluble in dilute nitric acid. Upon heating with alkalies, the yellow neutral salt is converted into basic red chromate of lead (2 Pb O, Cr 0 ̧). 8. If insoluble chromates are fused together with carbonate of soda and nitrate of soda, and the fused mass is treated with water, the fluid produced appears YELLOW from the alkaline chromate which it holds in solution; upon the addition of an acid the yellow color changes to red. The oxides remain behind either in the pure state or as carbonates, if they are not soluble in the caustic soda formed from the nitrate. $139. Remarks. When testing for bases, we always find the chromic acid as sesquioxide of chromium, since hydrosulphuric acid reduces it to that state. The characteristic color of the solution frequently renders the application of any further test unnecessary. The reactions with salts of silver and salts of lead afford positive and confirmatory proof of the presence of chromic acid in aqueous solutions. Acid solutions are boiled with carbonate of soda in excess, and filtered; the filtrate is acidified with acetic acid, and acetate of lead then added to it. If there is reason to suppose that chromic acid is present in a solution, metallic oxides being also present, it is preferable to effect the reduction of the chromic acid by means of hydrochloric acid and alcohol, or by sulphurous acid, instead of reducing it by hydrosulphuric acid. § 140. Supplement to the First Division of the First Group. a. Selenious acid (Se O). Sublimed anhydrous selenious acid appears in form of white four-sided needles, its hydrate in form of crystals resembling those of nitrate of potassa. Both the acid and its hydrate dissolve readily in water to a strongly acid fluid. Of the neutral salts only those with the alkalies are soluble in water. All selenites dissolve readily in nitric acid, with the exception of the selenites of lead and silver, which dissolve with difficulty. Hydrochloric acid, even when boiling, fails to decompose selenites; sulphuric acid decomposes them readily with the aid of heat. Hydrosulphuric acid produces in solutions of selenious acid or of selenites (in presence of free hydrochloric acid) a yellow precipitate of SULPHIDE OF SELENIUM, which, upon heating, turns reddish-yellow, and is soluble in sulphide of ammonium. Chloride of barium produces (after neutralization of the free acid, should any be present) a white precipitate of selenite of baryta, which is soluble in hydrochloric acid and in nitric acid. Protochloride of tin produces a red precipitate of SELENIUM, which turns gray at a high temperature ; sulphurous acid produces the same precipitate. When exposed on a charcoal support to the reducing flame, the selenites evolve selenium, exhaling at the same time a characteristic odor of decaying horseradish. b. Sulphurous acid (SO) is a colorless, uninflammable gas, which exhales the stifling odor of burning sulphur. It dissolves copiously in water. The solution has the odor of the gas, reddens litmus paper, and bleaches Brazil-wood paper. It absorbs oxygen from the air, and is |