below the circular aperture, a tube to which is attached some vulcanized india-rubber, which serves to convey the gas to the apparatus. This tube is turned as shown in Fig. 15; it has a bore of 4 millimètres diameter. The gas conveyed into it through the india-rubber re-issues from a tube placed in the centre of the cavity of the box. This tube, which is 4 millimètres thick at the top, thicker at the lower end, projects 3 millimètres above the rim of the box; the gas issues from a narrow opening which appears formed of 3 radii of a circle, two of them forming an angle of 120° with one another. The length of each radius is 1 millimètre, the opening millimètre wide; ef is a brass tube 75 millimètres long, open at both ends, and having an inner diameter of 9 millimètres; the screw at the lower end of this tube fits into a nut in the upper part of the cavity of the box. With this tube screwed in, the lamp is completed. On opening the stop-cock, the gas rushes from the opening into the tube ef, when it mixes with the air coming in through the circular apertures (c). When this mixture is kindled at f, it burns with a straight, bluish flame, entirely free from soot, which may be regulated at will by opening the stop-cock more or less; tial opening of the cock suffices to give a flame fully answering the purpose of the common simple spirit lamp; whilst, with the full stream of gas turned on, the flame, which will now rise up to 2 decimètres in height, burning with a roaring noise, affords a most excellent substitute for the Berzelius lamp. Flasks, &c., which it is intended to heat over

a

the gas lamp, are most conveniently supported on wire gauze. If it is wished to use the gas lamp for blowpipe operations, the tube g h must be inserted into ef; this tube terminates in a flattened top slanting at an angle of 68°, to the axis, and having an opening in it 1 centimètre long and 1 to 2 millimètres wide; its insertion into e f serves to close up the airapertures in the box, and pure gas, burning with luminous flame, issues cordingly now from the top of the tube. Fig. 16 shows the apparatus complete, fixed in the forked iron stand; this arrangement permits the lamp being moved backward and forward between the prongs of the fork, and up and down the pillar of the stand. The moveable ring on the same pillar serves to support the object to be opeThe 6 radii round the tube of the lamp serve to support a porcelain plate used in quantitative analyses.

rated upon.

Fig. 16.

ac

3. A BLOWPIPE (see § 13).

4. A PLATINUM CRUCIBLE which will contain about a quarter of an ounce of water, with a cover of the form of a shallow dish; it must not be too deep in proportion to its breadth.

5. PLATINUM FOIL, as smooth and clean as possible, and not very thin length about 40 millimètres; width about 25 millimètres.

6. PLATINUM WIRE (see § 13); three or four wires are amply sufficient. They are kept most conveniently in a glass filled with water, most of the beads being dissolved by that fluid when left in contact with it for some time; the wires may thus be always kept clean.

7. A STAND WITH TWELVE TEST TUBES-16 to 18 centimètres is about the proper length of the tubes, from 1 to 2 centimètres the proper width. The pegs on the

upper shelf serve for the clean tubes, which may thus be always kept dry and ready for use. The tubes must be made of thin white glass, and so well annealed that they do not crack even though boiling water be poured into them. The rim must be quite round, and slightly turned over; it ought not to have a lip, as this is useless, simply preventing the tube being closely stopped with the finger, and also shaking the contents.

8. SEVERAL BEAKER GLASSES AND SMALL RETORTS of thin, well annealed glass.

9. SEVERAL PORCELAIN EVAPORATING DISHES, AND A VARIETY OF SMALL PORCELAIN CRUCIBLES. Those of the royal manufacture of Berlin are unexceptionable, both in shape and durability.

10. SEVERAL GLASS FUNNELS of various sizes. They must be inclined at an angle of 60°, and merge into the neck at a definite angle.

11. A WASHING BOTTLE of a capacity of from 300 to 400 cubic centimètres (see § 6).

12. SEVERAL GLASS RODS AND GLASS TUBES. The latter are bent, drawn out, &c., over a Berzelius spirit lamp; the former are rounded at the ends before the blowpipe.

13. A selection of WATCH-GLASSES.

14. A small AGATE MORTAR.

15. A pair of small STEEL or BRASS PINCERS, about four or five inches long.

16. A WOODEN FILTERING STAND (see § 5).

17. A TRIPOD of thin iron, to support the dishes, &c., which it is intended to heat over the small spirit or gas lamp.

SECTION II.

REAGENTS.

§ 15.

A VARIETY of phenomena may manifest themselves upon the decomposition or combination of bodies. In some cases liquids change their color, in others precipitates are formed; sometimes effervescence takes place, and sometimes deflagration, &c. Now, if these phenomena are very striking, and attendant only upon the action of two definite bodies upon one another, it is obvious that the presence of one of these bodies may be detected by means of the other: if we know, for instance, that a white precipitate of certain definite properties is formed upon mixing baryta with sulphuric acid, it is clear that, if upon adding baryta to any liquid, we obtain a precipitate exhibiting these properties, we may conclude that this liquid contains sulphuric acid.

Those substances which indicate the presence of others by any striking phenomena are called reagents.

According to the different objects attained by their application we make a distinction between general and special reagents. By general reagents we understand those which serve to determine the class or group to which a substance belongs; and by special reagents those which serve to detect and determine bodies individually. That the line between the two divisions cannot be drawn with any degree of precision, and that one and the same substance is often made to serve both as a general and a special reagent, cannot well be held a valid objection to this classification, which, in fact, is simply intended to induce a habit of employing reagents always for a settled purpose-viz., either simply to find out the group to which the substance under examination belongs, or to determine the latter individually.

Now whilst the usefulness of general reagents depends principally upon their efficiency in strictly characterizing groups of bodies, and often effecting a complete separation of the bodies belonging to one group from those belonging to another, that of special reagents depends upon their being characteristic, and upon their being 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 a characteristic reagent for copper, protochloride of tin for mercury, because the phenomena produced by these reagentsviz., 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, terchloride 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

We

constituents, and must contain no admixture of foreign substances. 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 admixture.

9966

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, when 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. The 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.

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 It is simply this: let the student always reflect, before the addition of a reagent, for what purpose he applies it, and what are the phenomena he intends to produce.

cases.

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 humid way; and 2, Reagents in the dry way. For greater clearness we subdivide these two principal classes as follows:—

A. REAGENTS IN THE HUMID WAY.

I. SIMPLE SOLVENTS.

II. ACIDS (and HALOGENS).

a. Oxygen acids.

b. Hydrogen acids and halogens.
c. Sulphur acids.

III. BASES (and METALS).

a. Oxygen bases.

b. Sulphur bases.

IV. SALTS.

a. Of the alkalies.

b. Of the alkaline earths.

c. Of the oxides of the heavy metals.

V. COLORING MATTERS AND INDIFFERENT VEGETABLE SUBSTANCES.

B. REAGENTS IN THE DRY WAY.

I. FLUXES.

II. BLOWPIPE REAGENTS.

A. REAGENTS IN THE HUMID 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 matter 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.)

§ 16.

1. 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 is 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. Pure distilled water must be colorless, inodorous, and tasteless, and must leave no residue upon evaporation in a platinum vessel. Sulphide of ammonium must not alter it (copper, lead, iron); its transparency must not be in the least impaired by basic acetate of lead (carbonic acid, carbonate of ammonia), nor, even after long standing, by oxalate of ammonia (lime), chloride of barium (sulphates), or nitrate of silver (metallic chlorides).

Uses. We use water* principally as a simple solvent for a great variety of substances; the most convenient way of using it is with the washing bottle (see § 6, Fig. 3), by which means a large or fine 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.

* In analytical experiments we use only distilled water; whenever, therefore, the term "water" occurs in the present work, distilled water is meant.