acid; but this decomposition gives rise to the formation or inso luble compounds when oxide of silver is present, and of sparingly soluble compounds in the presence of suboxide of mercury and lead. This is not the case with nitric acid, and accordingly the latter effects complete solution in many cases where hydrochloric acid has left a residue. But, on the other hand, nitric acid leaves, besides the bodies insoluble in any simple acid, teroxide of antimony, binoxide of tin, binoxide of lead, &c., undissolved, and fails also to effect the complete solution of many other substances, e. g. sesquioxide of iron and alumina.

Substances not soluble in water are, therefore, treated as follows: try to dissolve them in dilute or concentrated hydrochloric acid, cold or boiling; if this fails to effect complete solution, try to dissolve a fresh portion in nitric acid; if this also fails, treat the body with aqua regia, which is an excellent solvent, more particularly for metallic sulphides. To examine separately the solution in hydrochloric acid or in nitric acid, on the one hand, and that in nitrohydrochloric acid on the other, is, in most cases, unnecessary. To prepare a nitric or nitrohydrochloric solution is disadvan tageous when not necessary, because a hydrochloric solution is much better adapted for precipitation with hydrosulphuric acid. To evaporate an aqua-regia solution for the purpose of removing excess of acid is unadvisable, since volatile chlorides, especially chloride of arsenic, may thus be lost. Use, therefore, no more acid at the outset than is necessary in making the solution.

With regard to the solution of metals and alloys, I have to remark that, upon boiling them with nitric acid, white precipitates will frequently form, although neither tin nor antimony be present. Inexperienced students often confound such precipitates with the oxides of these two metals, although their appearance is quite different. These precipitates consist simply of nitrates sparingly soluble in the nitric acid present, but readily soluble in water. Consequently the analyst should ascertain whether these white precipitates will dissolve in water or not, before he concludes them to consist of tin or antimony.

III. ADDITIONAL REMARKS TO THE ACTUAL ANALYSIS.

To §§ 185-207.

A. GENERAL REVIEW AND EXPLANATION OF THE ANALYTICAL

COURSE.

a. DETECTION OF THE BASES.

The classification of the bases into six groups, and the methods which serve to detect and isolate them individually, have been fully

*For the exceptions, see 206.

explained in Part I., Section III. The systematic course of analysis, from § 192 to § 201, is founded upon this classification of the bases; and as a correct apprehension of it is of primary importance, I will here subjoin a brief explanation of the grounds upon which this division rests. Respecting the detection of the several bases individually, I refer the student to the recapitulations and remarks in §§ 95, 102, 106, 115, 121, 126, 130 and 137.

The general reagents which serve to divide the bases into principal groups are-HYDROCHLORIC ACID, HYDROSULPHURIC ACID, sulPHIDE OF AMMONIUM, and CARBONATE OF AMMONIA; this is likewise the order of succession in which they are applied. Sulphide of ammonium performs a double part.

Let us suppose we have in solution the whole of the bases, together with arsenious and arsenic acids, and also phosphate of lime-which latter may serve as a type for the salts of the alkaline earths, soluble in acids, and reprecipitated unaltered by ammonia.

Chlorine forms insoluble compounds only with silver and mercury; chloride of lead is sparingly soluble in water. The insoluble subchloride of mercury corresponds to the suboxide of that metal. If, therefore, we add to our solution:

1. Hydrochloric Acid,

we remove from it the metallic oxides of the first division of the fifth group, viz. the whole of the OXIDE OF SILVER and the whole of the SUBOXIDE OF MERCURY. From concentrated solutions, a portion of the LEAD may likewise precipitate as chloride; this is, however, immaterial, as a sufficient quantity of the lead remains in the solution to permit the subsequent detection of this metal.

Hydrosulphuric acid completely precipitates the oxides of the fifth and sixth group from solutions containing a free mineral acid, since the affinity of the metallic radicals of these oxides for sulphur, and that of the hydrogen for oxygen, are sufficiently powerful to overcome the affinity between the metal and the oxygen, and that between the oxide and a strong acid, EVEN THOUGH THE ACID BE PRESENT IN EXCESS. But none of the other bases are precipitated under these circumstances, since those of the first, second, and third group form no sulphur compounds insoluble in water; and the affinity which the metallic radicals of the oxides of the fourth group possess for sulphur, combined with that manifested by hydrogen for oxygen, is not sufficiently powerful to overcome the affinity of the metal for oxygen and of the oxide for a strong acid, If the

LATTER IS PRESENT IN EXCESS.

If, therefore, after the removal of the oxide of silver and suboxide of mercury, by means of hydrochloric acid, we add to the solution, which still contains free hydrochloric acid,

2. Hydrosulphuric Acid,

we remove from it the remainder of the oxides of the fifth, toge ther with those of the sixth group, viz., OXIDE OF LEAD, OXIDE OF MERCURY, OXIDE OF COPPER, TEROXIDE of bismuth, OXIDE OF CADMIUM, TEROXIDE OF GOLD, BINOXIDE OF PLATINUM, PROTOXIDE OF TIN, BINOXIDE OF TIN, TEROXIDE OF ANTIMONY, ARSENIOUS ACID, and ARSENIC ACID. All the other oxides remain in solution, either unaltered, or reduced to a lower degree of oxidation, e. g. sesquioxide of iron to protoxide; chromic acid to sesquioxide of chromium, &c. The sulphides corresponding to the oxides of the sixth group combine with basic metallic sulphides (the sulphides of the alkali metals), and form with them sulphur salts soluble in water; while the sulphides corresponding to the oxides of the fifth group do not possess this property, or possess it only to a limited extent." therefore, we treat the whole of the sulphides precipitated by hydrosulphuric acid from an acid solution, with

* If,

3. Sulphide of Ammonium (or, in certain cases, Sulphide of Sodium),

if necessary, with addition of some sulphur or yellow sulphide of ammonium, the sulphides of mercury, lead, copper, bismuth, and cadmium remain undissolved, whilst the other sulphides dissolve as double compounds of sulphide of GOLD, PLATINUM, ANTIMONY, TIN, ARSENIC, with SULPHIDE OF AMMONIUM (or, as the case may be, SULPHIDE OF SODIUM), and precipitate again from this solution upon the addition of an acid, either unaltered, or, as regards the protosulphide of tin and the tersulphide of antimony, in a state of higher sulphuration-these two compounds taking up sulphur from the yellow sulphide of ammonium. The rationale of this precipitation is as follows:-The acid decomposes the sulphur salt formed. The sulphur base (sulphide of ammonium or sulphide of sodium) transposes with the constituents of the water, forming an oxygen base (oxide of ammonium or soda) and hydrosulphuric acid; the former combines with the acid added, the latter escapes, and the liberated electro-negative sulphide (sulphur acid) precipitates. If the acid is an hydracid, its radical combines with the ammonium, its hydrogen with the sulphur. Sulphur precipitates at the same time, the sulphide of ammonium containing generally an excess of that element. The analyst must bear in mind that this eliminated sulphur makes the precipitated sulphides appear of a lighter color than they are naturally.

The alkalies, the alkaline earths, alumina, and sesquioxide of

*Sulphide of mercury combines with sulphide of potassium and sulphide of sodium, but not with sulphide of ammonium; sulphide of copper dissolves a little ir sulphide of ammonium, but not in sulphide of potassium or sulphide of sodium.

chromium have remained in solution, because their sulphur com pounds are soluble in water, or because their salts are not affected in the least by hydrosulphuric acid; the sulphides corresponding to the oxides of the fourth group are insoluble in water, and would have been precipitated accordingly by hydrosulphuric acid, but for the free acid present. If, therefore, this free acid is removed, i. c., if the solution is made alkaline, and then treated with hydrosulphuric acid, or, what answers both purposes at once, if

4. Sulphide of Ammonium,

is added to the solution,* the sulphides corresponding to the oxides of the fourth group will precipitate: viz., the SULPHIDES OF IRON, MANGANESE, COBALT, NICKEL, and ZINC. But in conjunction with them, ALUMINA, SESQUIOXIDE OF CHROMIUM, and PHOSPHATE OF LIME are thrown down, because the affinity which the oxide of ammonium possesses for the acid of the salt of alumina or of sesquioxide of chromium, or for that which keeps the phosphate of lime in solution, causes the elements of the sulphide of ammonium to transpose with those of the water, thus giving rise to the formation of oxide of ammonium and of hydrosulphuric acid. The former combines with the acid, the latter escapes, being incapable of entering into combination with the liberated oxides or with the phosphate of lime, the oxides and the lime-salt precipitate.

There remain now in solution only the alkaline earths and the alkalies. The neutral carbonates of the former are insoluble in water, whilst those of the latter are soluble in that menstruum. If, therefore, we now add

5. Carbonate of Ammonia,

and some ammonia, in order to prevent the formation of bicarbonates, the whole of the alkaline earths ought to precipitate. This is, however, the case only as regards BARYTA, STRONTIA, and LIME; of magnesia we know that, owing to its disposition to form double compounds with salts of ammonia, it precipitates only in part; and that the presence of an additional salt of ammonia will altogether prevent its precipitation. To guard against any uncertainty arising from this cause, chloride of ammonium is added previously to the addition of the carbonate of ammonia, and thus the precipitation of the magnesia is altogether prevented.

We have now in solution magnesia and the alkalies. The detection of magnesia may be effected by means of phosphate of soda

After previous neutralization of the free acid by ammonia, to prevent unneces sary evolution of hydrosulphuric acid: and the addition also, if necessary, of chlorid of ammonium to prevent the precipitation of magnesia by ammonia.

and ammonia; but its separation requires a different method, since the presence of phosphoric acid would impede the further progress of the analysis. The process which serves to effect the removal of the magnesia is based upon the insolubility of that earth in the pure state. The substance under examination is accordingly ignited in order to expel the salt of ammonia, and the magnesia is then precipitated by means of baryta, the alkalies, together with the newly formed salt of baryta and the excess of the caustic baryta added, remaining in solution. By the addition of carbonate of ammonia, the compounds of baryta are removed from the solution, which now only contains the fixed alkalies, the salt of ammonia formed, and the excess of the salt of ammonia added. If the salts of ammonia are then removed by ignition, the residue consists of the fixed alkalies alone. This method of separating the baryta affords the advantage over that of effecting the removal of that earth by means of sulphuric acid, that the alkalies are obtained in the most convenient form for their subsequent individual detection and isolation, viz., as chlorides. But as carbonate of baryta is slightly soluble in salts of ammonia, and gives, upon evaporation with chloride of ammonium, carbonate of baryta and chloride of barium, it is usually necessary, after the expulsion of the salts of ammonia by ignition, to precipitate it once more with carbonate of ammonia, in order to obtain a solution perfectly free from baryta. Lastly, to effect the detection of the ammonia, a fresh portion of the substance must of course be taken.

b. DETECTION OF THE ACIDS.

Before passing on to the examination for acids and salt radicals, the analyst should first ask himself which of these substances may be expected to be present, from the nature of the detected bases and the class to which the substance under examination belongs with respect to its solubility in water or acids, since this will save him the trouble of unnecessary experiments. Upon this point I refer the student to the table in Appendix IV., in which the various compounds are arranged according to their several degrees of solubility in water and acids.

The general reagents applied for the detection of the acids are, for the inorganic acids CHLORIDE OF BARIUM and NITRATE OF SILVER, for the organic acids CHLORIDE OF CALCIUM and SESQUICHLORIDE of IRON. It is therefore indispensable that the analyst should first assure himself whether the substance under examination contains only inorganic acids, or whether the presence of organic aids must also be looked for. The latter is invariably the case, if the body, when ignited, turns black, owing to separation of carbon.In the examination for bases the different reagents serve to effect the actual separation of the several groups of bases from each