when mixed with certain liquid or solid substances. Such substances, added for analytical purposes, are called reagents. Some reagents serve for detecting one element or compound only, and are therefore called special reagents; others separate or detect a group of elements or compounds, and are called group reagents or general reagents: these latter reagents serve to separate substances into "Analytical Groups," as will be more fully explained hereafter.

The student must bear in mind that, although an acquaintance with the most important reaction of substances is indispensable to success as an analyst, caution, skill, and neatness in manipulation are not less necessary, and the most scrupulous attention must be paid to all directions bearing on these matters.

36. Analytical Classification.—The chemical elements are frequently divided into two classes-viz., metals and nonmetals. These classes form, with a slight modification, convenient divisions for analytical purposes. In the class of "metals" must be included the hypothetical metallic radicle ammonium (NH4), also the element hydrogen. The class of "non-metals," either singly or in groups formed by their combination inter se or less frequently with a metal, unite with hydrogen and with metals forming "salts"; the hydrogen salts of these radicals being often called "acids." Thus C1,* SO4, PO4, MnO4 form the hydrogen salts (acids), HCl, H2SO4, H3PO4, HMnO4 and the metallic salts, KCl, MgSO4, NaPO4, KMnO4: metal hydrogen salts (acid-salts), such as NaHSO4, Na,HPO4, are also known. The general name "acid radicle" is conveniently applied to Cl, SO4, PO4 MnO4, &c., it being understood that although most of these are unisolated compound radicles,-i.e., contain more than one element-some of them are elements; their common property is that of forming in combination with hydrogen and with metals-salts. For analytical purposes, then, we

* The meaning and use of chemical symbols will briefly be explained hereafter (40); but the student is presumed to be already acquainted with them. In par. 540 will be found a list of the symbols of the different elements.

E

may separate the substances we have to detect into the groups of Metals and Acid Radicles. In trying the reactions for these substances, it is usual to employ, not the substances themselves, but certain compounds-usually saltswhich contain them.

For convenience of hasty writing, it is not uncommon to employ abbreviations for the compound radicles, especially for such as are constantly met with or are of unusual complexity. In the text the only such abbreviations employed

are:

Am for (NH4), the metallic radicle ammonium.

T

A

Cfy

Ho

(CHO), the acid radicle of tartaric acid and the tartrates.

(C2H3O2), the acid radicle of acetic acid and the

acetates.

(FeCy), the acid radicle of ferro- and ferri-cyanides. ,, (HO), the radicle Hydroxyl, occurring in hydrates,

&c.

37. Analytical Groups.-The metals are separated by their behaviour with certain general reagents into five principal groups, two of these being further subdivided into two subgroups. Each of these groups receives a distinctive name— either a number from its place in the systematic course, or a name from some conspicuous or important member of the group, or from the general reagent used to precipitate the group. Thus we speak indifferently of the 4th Group, the Barium Group, or the Ammonium-carbonate Group. On pages 168 and 169 will be found a table showing the analytical groups with their distinctive numbers, names, and group reagents placed at the head of each column; and the names and symbols of the elements contained in the group arranged vertically beneath.

It is perhaps well to avoid referring to a group by its number, as different analysts number the groups differently, and the name thus given is therefore apt not to denote the group with certainty.

38. Method of trying the Analytical Reactions in Section IV. In order to become fully acquainted with the behaviour

of different substances with "reagents" and thus to be able to detect them by their "reactions," each of these reactions should be performed with the substance itself or with one of its compounds, and the appearances presented carefully observed, so that the substance would be easily recognised in a body of unknown composition by means of those tests.

The following general rules must be carefully attended to in trying the reactions; they may be read through before commencing the reactions par. 47:

1. A solution of the substance which is directed to be used must always be employed, unless it is specially stated that the solid is required. Solid substances are required as a rule only for blowpipe reactions. A solution can be readily made from the solid, if necessary, by warming it with water or in some cases with dilute acid (529). This is often called the "original solution."

2. Commence by taking in a test-tube or small beaker a small quantity (about 15-20 c.c.) of the solution of the substance. If the solid substance is also required, take some of it upon a watch-glass.

3. Pour a small portion of the solution into a perfectly clean test-tube, and add the first-mentioned reagent to it. The reagent must always be added in small quantity at first; more can be employed afterwards if excess is required. The student must from the first acquire the habit of working with small quantities both of solutions and reagents.

For each of the reactions a separate portion of the “original solution" is thus poured into a clean test-tube and the required reagent added to it.

4. Before trying each reaction the student must carefully read through the description of the results he is to obtain; he must then perform the experiment literally following out the directions given. He must consider it a necessary condition of after success that each result is obtained precisely as stated in the text, and must never on any account pass on until he has conscientiously satisfied himself that the statements of the book are true, and that he could at any time repeat the test successfully.

39. Entry in Note-Book.-Each reaction, as soon as it has been satisfactorily tried, should be neatly entered in the notebook in a short form. Since nothing so much aids brevity as the symbolic notation commonly used by chemists, the student should invariably adopt this chemical shorthand in entering results. A full account and explanation of chemical notation may be found in any work on theoretical chemistry, and merely a few hints will therefore be here given which will be of special use for the purpose of briefly entering reactions.

40. Chemical Notation.—Each element has its corresponding symbol consisting of one or two letters; thus S stands for sulphur, Cl for chlorine. These symbols also stand for one atom of each element; by being written one after another they give the formula of a compound substance, showing what elements, and how many atoms of each of these elements, its molecule contains: thus KCl stands for one molecule of potassium chloride, a compound of one atom of potassium and one atom of chlorine; PtCl stands for one molecule of platinic chloride, which consists of one atom of platinum combined with four atoms of chlorine, the small figure placed below a symbol to the right denoting how many atoms of that element are present. A number placed before a formula multiplies each symbol in that formula; thus 2PtCl signifies two molecules of platinum chloride, containing together two atoms of platinum and eight atoms of chlorine. If two or more symbols are included in brackets any number placed outside, either before or below the brackets multiplies each symbol contained in the brackets; thus both Sr(NO), and Sr2(NO3) stand for SrN2O, and this may also be written Sr. 2NO. In paragraph 540 will be found a list of the elements, with their symbols and atomic weights; paragraphs 524, 525, 526, 529, and 530 contain a list of the names and corresponding formula of all the most common compound substances mentioned in the course of the book. Symbols and formulæ are always used in the text instead of names, but the student can readily find for these corresponding names by reference to the above tables, or to the labels on the bottles which should contain both names and formulæ.

41. On adding a "reagent" to a substance some change usually occurs which gives rise to an alteration of colour, or very frequently to the formation of a precipitate having a certain colour, appearance, or behaviour with other liquids, which are characteristic of that particular substance, or of some element or group of elements contained in it; this change should be briefly described in the note-book in words, and then represented by a chemical equation. The rules for drawing out an equation may be found in any treatise on chemistry; a few general directions only are here given which will be of service for the special object in question.

42. Rules for writing down an Equation.-Write down the formulæ of the two substances which are mixed together for the reaction, with the sign of addition (+) between them; then write the sign of equality (=), followed by the formula of the precipitate produced. In a completed equation the formula written on the left-hand side of the sign (=) are usually called the "left-hand side" of the equation; those to the right the "right-hand side." Since most of the following reactions are cases of "double decomposition"-i.e., cases where an exchange of certain elements or groups of elements occurs between the two compounds,—a little consideration will usually show how many molecules of the substances on the left-hand side of the equation are required to yield the formula for the precipitate, and also whether any other, and if so what other, substance is at the same time formed.

It is always necessary that the number of atoms of any one element on both sides of the equation should be equal. The following are dissected examples :—

This is simply placing together the substances mixed and the precipitate formed; the equation thus formed is evidently not correct, since we have two atoms of K on the right-hand side and only one on the left, also 6 atoms of Cl on the right and only 5 on the left; but this inequality is at once removed if we place 2 before the KCl, thus:

2KCl + PtCl = K ̧PtCl ·