ticable, an approximate estimation of the respective quantities of the several constituents, it is usually the best way to examine separately; (1) the part soluble in water; (2) the part soluble in hydrochloric acid; and (3) the residue which is insoluble in either menstruum. This can be done the more readily, as the number of bodies to which regard must be had in the analysis is only small, and the several processes may accordingly be expeditiously performed.

a. Examination of the Part Soluble in Water.

Boil the ash with water, filter, and whilst the residue is being washed, examine the solution as follows:

1. Add to a portion, after heating it, hydrochloric acid in 327 excess, warm, and let the fluid stand at rest. Effervescence indicates CARBONIC ACID, combined with alkalies; odor of hydrosulphuric acid indicates the SULPHIDE of an ALKALI METAL, formed from an alkaline sulphate by the reducing action of the carbon. Turbidity from separation of sulphur, with odor of sulphurous acid, denotes a HYPOSULPHITE (which occurs occasionally in the ash of coal). Filter, if necessary, and add to the filtrate or to the fluid if no filtration is required-some chloride of barium; the formation of a white precipitate indicates the presence of SULPHURIC acid.

2. Evaporate another portion of the solution until it is 328 reduced to a small volume, add hydrochloric acid to acid reaction-effervescence indicates the presence of CARBONIC ACID -evaporate now to dryness, and treat the residue with hydrochloric acid and water. The portion left undissolved consists of SILICIC ACID. Filter, add ammonia, chloride of ammonium, and sulphate of magnesia; the formation of a white precipitate indicates the presence of PHOSPHORIC acid. Instead of this reaction, you may also mix the fluid filtered from the silicic acid with acetate of soda, and then cautiously add, drop by drop, sesquichloride of iron; or you may test with molybdate of ammonia (§ 145).

3. Add to another portion of the solution nitrate of silver 329 as long as a precipitate continues to form; warm gently, and then cautiously add ammonia; if a black residue is left, this consists of sulphide of silver, proceeding from the sulphide of an alkali metal, or from a hyposulphite. Mix the ammoniacal solution now-after previous filtration if necessary-with nitric acid in slight excess, so that phosphate of silver, which at first falls, is redissolved, and only CHLORIDE (iodide and

*To detect the iodine in aquatic plants, dip the plant in a weak solution of potassa (Chatin), dry, incinerate, treat with water, and examine the aqueous solution as directed (257).

bromide) OF SILVER remains. Filter off the precipitate, which is to be further examined according to (178) and neutralize the filtrate cautiously and exactly with ammonia. If this produces a bright yellow precipitate, phosphoric acid is present in the tribasic; if a white precipitate, it is present in the bibasic form.

4. Acidify a portion of the solution with hydrochloric acid, 330 and then make it alkaline with ammonia; mix the alkaline fluid with oxalate of ammonia, and let it stand at rest. The formation of a white precipitate indicates LIME. Filter, and mix the filtrate with ammonia and phosphate of soda; the formation of a crystalline precipitate, which often becomes visible only after long standing, indicates MAGNESIA. Magnesia is often found in distinctly appreciable lime, only in exceedingly minute quantity, even when alkaline carbonates and phosphates are present.

5. For POTASSA and SODA examine as directed § 200. If magnesia is present, neutralize that portion of the solution intended for detecting alkalies with hydrochloric acid, and separate the magnesia as directed § 199, 2.

6. LITHIA, which is more frequently an ingredient of ashes than has been hitherto suspected, and RUBIDIA, which nearly always accompanies potassa, may be detected by the spectroscope (§ 96), in the residue of alkaline salts.

b. Examination of the Part Soluble in Hydrochloric Acid. Warm the residue left undissolved by water with hydro- 331 chloric acid-effervescence indicates CARBONIC ACID, combined with alkaline earths; evolution of chlorine denotes OXIDES OF MANGANESE-evaporate to dryness, and heat a little more strongly, to effect the separation of the silicic acid: moisten the residue with hydrochloric acid and some nitric acid, add water, warm, and filter.

1. Test with hydrosulphuric acid. If this produces any other than a perfectly white precipitate, you must examine it in the usual way. The ashes of plants occasionally contain COPPER; if the plant has been manured with excrements deodorized by nitrate of lead, they may contain LEAD [if with superphosphate made from arsenical oil of vitriol, arsenic, J. Dary].

2. Mix a portion of the original solution with carbonate of 332 soda, as long as the precipitate formed redissolves upon stirring; then add acetate of soda, and some acetic acid. This produces, in most cases, a white precipitate of PHOSPHATE OF SESQUIOXIDE OF IRON. If the fluid in which this precipitate

If the residue still contains much carbon, after further incineration.

is suspended is reddish, there is more sesquioxide of iron present than corresponds to the phosphoric acid; if it is colorless, add sesquichloride of iron, drop by drop, until the fluid looks reddish. (From the quantity of the precipitate of phosphate of sesquioxide of iron formed you may estimate the PHOSPHORIC ACID present.) Heat to boiling,* filter hot, and mix the filtrate, after addition of ammonia, with yellow sulphide of ammonium, in a stoppered flask; should a precipitate form, after long standing, examine this according to (141) for MANGANESE and ZINC,† and the fluid filtered from it for LIME and MAGNESIA, in the usual way (330).

3. To examine for BARYTA and STRONTIA, add dilute sulphuric acid to a portion of the hydrochloric solution, let the mixture stand for a considerable time, and test any precipitate that may form as directed (254).

c. Examination of the Residue Insoluble in Hydrochloric Acid. The residue insoluble in hydrochloric acid contains,

1. The silicic acid, which has separated on treating with 333 hydrochloric acid.

2. Those ingredients of the ash which are insoluble in hydrochloric acid. These are, in most ashes, sand, clay, carbon; substances, therefore, which are present in consequence of defective cleaning or imperfect combustion of the plants, or matter derived from the crucible. It is only the ashes of the stems of cereals and others abounding in silicic acid, that are not completely decomposed by hydrochloric acid.

Boil the washed residue with solution of carbonate of soda 334 in excess, filter hot, wash with boiling water, and test for silicic acid in the filtrate by evaporation with hydrochloric acid (§ 153, 2). If the ash was of a kind to be completely decomposed by hydrochloric acid, the analysis may be considered as finished-for the accidental admixture of clay and sand will rarely interest the analyst sufficiently to warrant a more minute examination by fluxing. But, if the ash abounded in silicic acid, and it may therefore be supposed that the hydrochloric acid has failed to effect complete decomposition, evaporate half of the residue insoluble in solution of carbonate of soda, with pure solution of soda in excess, in a silver or platinum dish, to dryness. This decomposes the silicates of the ash, whilst but little affecting the sand. Acidify now with hydrochloric acid, evaporate to dryness, &c., and proceed as in (331). For the detection of the alkalies use the other half of the residue (228).

If this should fail to decolorize the fluid, add some more acetate of soda. [Zinc is found in a species of violet growing in the neighborhood of zinc mines]

SECTION III.

EXPLANATORY NOTES AND ADDITIONS TO THE SYSTEMATIC COURSE OF ANALYSIS.

I. ANDIONAL REMARKS TO THE PRELIMINARY EXAMINATION.

To §§ 178-181.

THE inspection of the physical properties of a body may, as already stated, § 178, in many cases enable the analyst to draw certain general inferences as to its nature. Thus, for instance, if the analyst has a white substance before him, he may at once conclude that it is not cinnabar, or if a light substance, that it is not a compound of lead, &c.

Inferences of this kind are quite admissible to a certain extent; but if carried too far, they are apt to mislead the operator, by blinding him to every reaction not exactly in accordance with his preconceived notions.

As regards the examination of substances at a high temperature, platinum foil or small iron spoons may also be used in the process; however, the experiment in the glass tube gives, in most cases, results more clearly evident, and affords, moreover, the advantage that volatile bodies are less likely to escape detection, and that a more correct and precise notion can be formed of the nature of the heated substance, than exposure on platinum foil or in an iron spoon will permit. To ascertain the products of oxidation of a body, it is sometimes advisable also to heat it in a short glass-tube, open at both ends, and held in a slanting position; small quantities of a metallic sulphide, for instance, may be readily detected by this means. (Compare § 159, 6.)

With respect to the preliminary examination by means of the blowpipe, I have to remark that the student must avoid drawing positive conclusions from pyrochemical experiments, until he has acquired some practice in this branch of analytical chemistry. A slight incrustation of the charcoal support, which may seem to denote the presence of a certain metal, is not always a conclusive proof of the presence of that metal; nor would it be safe to assume the absence of a substance simply because the blowpipe flame fails to effect reduction, or solution of nitrate of protoxide of cobalt fails to impart a color to the ignited mass, &c. The blowpipe reactions are, indeed, in most cases, unerring, but it is not always easy to produce them, and they are moreover liable to suffer modification by accidental circumstances.

The student should never omit the preliminary examination; the notion that this omission will save time and trouble, is very errone

ous. The beginner may, for example, spend hours in searching for organic acids, when the simple preliminary test would show that they are all absent.

II. ADDITIONAL REMARKS TO THE SOLUTION OF SUBSTANCES, ETC

To §§ 182-184.

It is a task of some difficulty to fix the exact limit between sub. stances which are soluble in water and those that are insoluble in that menstruum, since the number of bodies which are sparingly soluble in water is very considerable, and the transition from sparingly soluble to insoluble is very gradual. Sulphate of lime, which is soluble in 430 parts of water, might perhaps serve as a limit between the two classes, since this salt may still be positively detected in aqueous solution, by the delicate reagents which we possess for lime and sulphuric acid.

When examining an aqueous fluid by evaporating a few drops of it upon platinum foil, to see whether it holds a solid body in solution, a very minute residue sometimes remains, which leaves the analyst in doubt respecting the nature of the substance; in cases of the kind test, in the first place, the reaction of the fluid with litmus papers; in the second place, add to a portion of it a drop of solution of chloride of barium; and lastly, to another portion some carbonate of soda. Should the fluid be neutral, and remain unaltered upon the addition of these reagents, the analyst need not examine it any further for bases or acids; since if the fluid contained any of those bases or acids which principally form sparingly soluble compounds, the chloride of barium and the carbonate of soda would have revealed their presence. The analyst may therefore feel assured that the detection of the substance of which the residue left upon evaporation consists, will be more readily effected in the class of bodies insoluble in water.

If water has dissolved any part of the substance under examination, the student will always do well to examine the solution both for acids and bases, since this will lead more readily to a correct apprehension of the nature of the compound—an advantage which will amply counterbalance the drawback of sometimes meeting with the same substance both in the aqueous and in the acid solution.

The following substances (with few exceptions) are insoluble in water, but soluble in hydrochloric acid or in nitric acid: the phosphates, arsenates, arsenites, borates, carbonates, and oxalates of the earths and metals; and also several tartrates, citrates, malates, benzoates, and succinates; the oxides and sulphides of the heavy metals; alumina, magnesia; many of the metallic iodides and cyanides, &c. Nearly the whole of these compounds are, indeed, decomposed, if not by dilute, by boiling concentrated hydrochlorio