from effecting a complete separation of the bodies belonging to one group those belonging to another, that of special reagents depends upon their being characteristic and sensitive. We call a reagent characteristic, if the alteration produced by it, in the event of the body tested for being present, is so distinctly marked as to admit of no mistake. Thus iron is à characteristic reagent for copper, protochloride of tin for mercury, because the phenomena produced by these reagents-viz., the separation of metallic copper and of globules of mercury-admit of no mistake. We call a reagent sensitive or delicate, if its action is distinctly perceptible, even though a very minute quantity only of the substance tested for be present; such is, for instance, the action of starch upon iodine. Very many reagents are both characteristic and delicate; thus, for instance, terchioride of gold for protoxide of tin; ferrocyanide of potassium for sesquioxide of iron and oxide of copper, &c. I need hardly mention that, as a general rule, reagents must be chemically pure-i.e., they must consist purely and simply of their essential constituents, and must contain no admixture of foreign substances. We must therefore make it an invariable rule to test the purity of our reagents before we use them, no matter whether they be articles of our own production or purchased. Although the necessity of this is fully admitted on all hands, yet we find that in practice it is too often neglected; thus it is by no means uncommon to see alumina entered among the substances detected in an analysis, simply because the solution of potassa used as one of the reagents happened to contain that earth; or iron, because the chloride of ammonium used was not free from that metal. The directions given in this section for testing the purity of the several reagents refer, of course, only to the presence of foreign matter resulting from the mode of their preparation, and not to mere accidental admixtures. One of the most common sources of error in qualitative analysis proceeds from missing the proper measure-the right quantity-in the application of reagents. Such terms as "addition in excess," supersaturation," &c., often induce novices to suppose that they cannot add too much of the reagent, and thus some will fill a test tube with acid, simply to supersaturate a few drops of an alkaline fluid, whereas every drop of acid added, after the neutralization point has once been reached, is to be looked upon as an excess of acid. On the other hand, the addition of an insufficient amount is to be equally avoided, since a reagent added in insufficient quantity often produces phenomena quite different from those which will appear if the same reagent be added in excess : e.g., a solution of chloride of mercury yields a white precipitate if tested with a small quantity of hydrosulphuric acid; but if treated with the same reagent in excess, the precipitate is black. Experience has, however, proved that the most common mistake beginners make, is to add the reagents too copiously. One reason why this over-addition must impair the accuracy of the results is obvious; we need simply bear in mind that the changes effected by reagents are perceptible within certain limits only, and that therefore they may be the more readily overlooked the nearer we approach these limits by diluting the fluid. Another reason is in the fact that a large excess of a reagent will often have a solvent or modifying action upon a precipitate or color, and will entirely prevent the exhibition of phenomena which a suitable quantity would without difficulty produce. No special and definite rules can be given for avoiding this source of error; a general rule may, however, be laid down, which will be found to answer the purpose, if not in all, at least in the great majority of cases. It is simply this: let the student always reflect before the addition of a reagent for what purpose he applies it, what are the phenomena he intends to produce, and what are the results of the addition of excess. We divide reagents into two classes, according to whether the state of fluidity, which is indispensable for the manifestation of the action of reagents upon the various bodies, is brought about by the application of heat, or by means of liquid solvents; we have consequently, 1, Reagents in the wet way; and 2, Reagents in the dry way. For greater clearness we subdivide these two principal classes as follows: A. REAGENTS IN THE WET WAY. I. SIMPLE SOLVENTS. II. ACIDS and HALOGENS. a. Oxygen acids. b. Hydrogen acids and halogens. III. BASES and METALS. a. Oxygen bases. b. Sulphur bases. IV. SALTS. a. Of the alkalies. b. Of the alkaline earths. e. Of the oxides of the heavy metals. V. COLORING MATTERS AND INDIFFERENT VEGETABLE SUB STANCES. B. REAGENTS IN THE DRY WAY. I. FLUXES. II. BLOWPIPE REAGENTS. A. REAGENTS IN THE WET WAY. I. SIMPLE SOLVENTS. Simple solvents are fluids which do not enter into chemical combination with the bodies dissolved in them; they will accordingly dissolve any quantity of mutter up to a certain limit, which is called the point of saturation, and is in a measure dependent upon the temperature of the solvent. The essential and characteristic properties of the dissolved substances (taste, reaction, color, &c.) are not destroyed by the solvent. (See § 2.) § 20. WATER (H O). Preparation. Pure water is obtained by distilling spring water from a copper still with head and condenser made of pure tin, or from a glass retort; which latter apparatus, however, is less suitable for the purpose. The distillation is carried to about three-fourths of the quantity operated upon. If it is desired to have the distilled water perfectly free from carbonic acid and carbonate of ammonia, the portions passing over first must be thrown away. In the larger chemical and in most pharmaceutical laboratories, the distilled water required ist obtained from the steam apparatus which serves for drying, heating, boiling, &c. Rain water collected in the open air may in many cases be substituted for distilled water.* Tests. It must be colorless, odorless, and tasteless, and should not leave the smallest residue when evaporated in a platinum vessel. It should not be changed by sulphide of ammonium (copper, lead, iron), nor rendered turbid by baryta water (carbonic acid). No cloudiness should be caused even after long standing by the addition of oxalate of ammonia (lime), of chloride of barium and hydrochloric acid (sulphuric acid), of nitrate of silver and nitric acid (chlorides), or of chloride of mercury and carbonate of soda (ammonia). Uses. We use watert principally as a simple solvent for a great variety of substances; the most convenient way of using it is with the washing bottle (see § 7, fig. 3), by which means a stronger or finer stream may be obtained. It serves also to effect the conversion of several neutral metallic salts (more particularly terchloride of antimony and the salts of bismuth) into soluble acid and insoluble basic compounds. § 21. 2. ALCOHOL (C ̧H ̧O2). == Preparation. Two sorts of alcohol are used in chemical analyses: viz., 1st, spirit of wine of 83 or 84 sp. gr. 91 to 88 per cent. by volume (spiritus rectificatus of the British Pharmacopoeia); and 2nd, absolute alcohol. The latter may be prepared most conveniently by mixing, in a distilling vessel, 1 part of fused chloride of calcium with 2 parts of rectified spirit of wine of about 90 per cent. by volume, digesting the mixture 2 or 3 days, until the chloride of calcium is dissolved, and then distilling slowly and in fractional portions. So long as the distillate shows a specific gravity below 810 (96-5 per cent. by volume), it may pass for absolute alcohol. The portions coming over after are received in a separate vessel. Tests.-Pure alcohol must completely volatilize, and ought not to leave the least smell of fusel oil when rubbed between the hands; nor should it alter the color of moist blue or red litmus paper. When kindled, it must burn with a faint bluish barely perceptible flame. As regards the preparation of water absolutely free from organic matter, see STAS, Zeitschrift f. anal. Chem., 6, 417. In analytical experiments we use only distilled water; whenever, therefore, the term water occurs in the present work, distilled water is meant. Uses.-Alcohol serves, (a) to effect the separation of bodies soluble in this fluid from others which do not dissolve in it, e.g. of chloride of strontium from chloride of barium; (b) to precipitate from aqueous solutions many substances which are insoluble in dilute alcohol, e.g. gypsum, malate of lime; (c) to produce various kinds of ether, e.g. acetic ether, which is characterized by its peculiar and agreeable smell; (d) to reduce, mostly with the co-operation of an acid, certain peroxides and metallic acids, e.g. binoxide of lead, chromic acid, &c.; (e) to detect certain substances which impart a characteristic tint to its flame, especially boracic acid, strontia, potassa, soda, and lithia. § 22. 3. ETHER (C,H ̧0). 4. CHLOROFORM (CHC). 5. SULPHIDE OF CARBON (CS). These solvents find but limited application in the qualitative analysis of inorganic bodies. They serve indeed almost exclusively to detect and isolate bromine and iodine. Chloroform and sulphide of carbon are preferable to ether in this respect. The latter is used for the detection of chromic acid by means of peroxide of hydrogen. These preparations are made much better on a large than on a small scale, and the best way therefore is to procure them by purchase. Tests.-Ether must have a specific gravity of 713 at 20°, and require 9 parts of water for solution. The solution must not alter the color of test papers. Ether must, even at the common temperature, rapidly and completely evaporate on a watch-glass. Chloroform must be colorless and transparent and have a specific gravity of 1:48. It must have no acid reaction, nor impair the transparency of solution of nitrate of silver. Mixed with 2 vols. of water, and shaken, its volume must not appear perceptibly diminished. It must even at the common temperature readily and completely evaporate on a watch-glass. Sulphide of carbon should be colorless, readily and completely volatile even at the common temperature, and exercise no action upon carbonate of lead. II. ACIDS AND HALOGENS. The acids--at least those of more strongly pronounced characterare soluble in water. The solutions taste acid and redden litmus paper. Acids are divided into oxygen acids, sulphur acids, and hydrogen acids. The oxygen acids, produced generally by the combination of a nonmetallic element with oxygen, combine with water in definite proportions to hydrated acids. It is with these hydrates that we have usually to do in analytical processes; they are contained in the aqueous solutions of the acids, and are commonly designated by the simple name of the free acid, as the accession of water does not destroy their acid properties. In the action of hydrated acids upon oxides of metals, the oxide takes the place of the water of hydration, and an oxygen salt is formed (HO, SO, + KO = KO, SO2+ HO). Where these salts are the pro duct of the combination of an acid with a strong base, their reaction (supposing the combining acid also to be a strong acid) is neutral; the salts formed with weaker bases, for instance, with the oxide of a heavy metal, generally show acid reaction, but are nevertheless called neutral salts if the oxygen of the base bears the same proportion to that of the acid in which it is found in the distinctly neutral salts of the same acid, or, in other terms, if it corresponds with the saturation capacity of the acid. Sulphate of potassa (K O, SO,) has a neutral reaction, whilst the reaction of sulphate of copper (Cu O, SO, + 5 aq.) is acid; yet the latter is nevertheless called neutral sulphate of copper, because the oxygen of the oxide of copper in it bears a proportion of 1 : 3 to that of the sulphuric acid, which is the same proportion as the oxygen of the potassa bears to that of the sulphuric acid in the confessedly neutral sulphate of potassa. = a The hydrogen acids are formed by the combination of the halogens with hydrogen. Most of these possess the characteristic properties o f acids in a high degree. They neutralize oxygen bases, with formation of haloid salts and water; HCl and Na O Na Cl and H 0,-3 H Cl and Fe, O, Fe, Cl, and 3 H O. The haloid salts produced by the action of powerful hydrogen acids upon strong bases have a neutral reaction; whilst the solutions of those haloid salts that have been produced by the action of powerful hydrogen acids upon weak bases (such as alumina and sesquioxide of iron) have an acid reaction. The sulphur acids are more frequently the result of the combination of metallic than of non-metallic elements with sulphur; they combine with sulphur bases to sulphur salts; HS+KS=KS, HS-As S, +3 Na S = 3 Na S, As S. The sulphur acids being weak acids, the soluble sulphur salts have all of them alkaline reaction. We use a. OXYGEN ACIDS. § 24. 1. SULPHURIC ACID (HO, SO). a. Concentrated sulphuric acid of commerce. b. Concentrated pure sulphuric acid. The following methods may be recommended for preparing chemically pure sulphuric acid: a. Put 1000 grm. of ordinary concentrated sulphuric acid in a porcelain dish, add 3 grm. of sulphate of ammonia, and heat till copious fumes of sulphuric acid begin to escape. This is done in order to destroy the oxides of nitrogen which are generally present in minute quantity. After cooling, add 4 or 5 grm. of coarsely powdered binoxide. of manganese, and heat to boiling with stirring, in order to convert any arsenious acid into arsenic acid. (BLONDLOT). When cool pour off the clear fluid by means of a long funnel tube into a coated retort. retort should not be more than half full, and is to be heated directly over charcoal. To prevent bumping, it is advisable to rest the retort on an inverted crucible cover, so that the sides may be more heated than the bottom. The neck of the retort must reach so far into the receiver that the acid distilling over drops directly into the body. To cool the receiver by means of water is unnecessary, and even dangerous. To The |