not only indicate the specific gravity, but also the composition and the quality of their baths; they imagine, for instance, that if they have once had good results with a bath marking 9°, any other bath also marking 9° must be equally good. This is a great mistake, which should be corrected.

A hydrometer, in general, is an instrument intended to indicate the specific gravity of a liquid, as compared with that of distilled water at its maximum density, that is, at 39.2° Fah. As there are liquids lighter and heavier than water, it is necessary to have two kinds of hydrometers, or rather, two different graduations of this instrument.

Baumé, an apothecary of Paris, was the first to construct a hydrometer of constant weight. It is a glass tube, the lower third of which is composed of a large bulb or cylinder, terminated by a smaller bulb, in which is placed a certain weight of lead or mercury acting as ballast for sinking the instrument more or less in the liquid. The cylindrical and narrow tube, above the large bulb, receives a small cylinder of paper upon which are marked the divisions, or degrees, of the scale. The liquid, the density of which is determined, is contained in a tall cylinder of glass, deep enough to permit the hydrometer to be immersed to its lowest point without touching bottom.

This instrument is graduated by two different methods, according as it is intended for determining the specific gravity of liquids heavier or lighter than water at 39.2° Fah.

If it be intended for liquids heavier than water (for acids and saline solutions, for instance), mercury or lead is introduced into the lower bulb until the instrument has sunk nearly to the top in pure water, and the 0 of the

paper scale is made level with that of the liquid. The hydrometer is then dipped into a solution of fifteen parts of common salt (chloride of sodium) in 85 parts of distilled water, and the line where the level of the liquid touches the tube is marked 15°. The space between 0 and 15° is divided into 15 equal parts, and this graduation is continued upon the remainder of the scale. 66° is the specific gravity of concentrated sulphuric acid with Baume's hydrometer.

Fig. 172.

A
-80

-70

60

50

For liquids lighter.than water, the graduation of the preceding hydrometer is reversed (Fig. 172). The 0 is at the lower part of the stem, and is determined by plunging the instrument into a solution of 10 parts of common salt in 90 parts of water. 10° is marked at the level line in distilled water, and the space between 0 and 10° is divided into 10 equal parts, and the same graduation continued for the remainder of the scale.

B

D

a

-40

30

-20

A similar method may be employed for constructing special hydrometers for special saline solutions, one of common salt, for instance. The 0 of the instrument is at the level line in distilled water; 5° in a solution of 5 parts of salt in 95 of water; 10° in a solution of 10 parts of salt in 90 of water; and so on.

But such an instrument cannot be employed for deter mining the composition of different solutions; and queer mistakes would be made if we desired to ascertain the composition of a solution of nitrate of silver with a hydrometer graduated especially for one of sulphite of soda, for instance. It is also evident that the same hydrometer will not do for determining the composition of a solution containing two different salts. Thus, a photographer may arrive at the composition of a pure solution of nitrate of

silver with a hydrometer especially constructed for the purpose; but the same instrument will give no indication whatever as to the proportion of silver contained in an electro-silvering bath, which contains many other salts besides that of silver.

Therefore, hydrometers are useful for verifying the strength of the commercial acids used, and to indicate whether the specific gravities of the baths are too much above or below the limits between which it has been ascertained that the galvanic current will pass freely.

Glass Balloons and Flasks.

Fig. 173.

These are spheres of thin blown glass (Fig. 173) with necks of various dimensions, in length and diameter. They are employed for heating acids, dissolving metals, and a great many other uses. They should be placed upon triangular supports of iron, and at a certain distance from the fire, from the direct action of which they should be protected by the intervention of a piece of wire gauze, or its equivalent. The thinner they are, the more easily they bear sudden changes of temperature. They are preferable to porcelain evaporating dishes, for dissolving gold, because there is much less danger of losing a part of the product by spurting.

Kettles and Boiling Pans.

These are of various shapes, hemispherical, or with flat bottom, and are made of different materials (Figs. 174, 175).

Those of copper are employed for whitening with silver and cream of tartar. Cast and sheet-iron are preferable for cleansing with caustic alkalies, or for evaporating residues.

Those of stoneware heated on sand or water baths, for desilvering (stripping), or for giving a dead lustre to copper in warm acids.

Cast-iron enamelled kettles are used for hot baths of copper, silver, gold, and platinum.

Notwithstanding their enamel, these vessels become gradually impregnated with the solutions they have held, and it is dangerous to employ them for different kinds of baths. Thus an enamelled kettle, which has been used for silvering, will not be suitable, even after the most thorough washing, for a gold bath, as the gilding will certainly be white or green, according to the quantity of silver retained by the vessel.

Brushes.

A definition of these instruments is unnecessary; and we shall simply indicate the various kinds suitable to the different operations of our art.

The fire-gilder employs, for equalizing the coating of amalgam, a long handled brush, the bristles of which are long and very stiff.

The electro-gilder uses a brush (Fig. 176), with long and flexible bristles.

For scouring with sand and pumice-stone alloys containing nickel, such as maillechort and German-silver,

which are difficult to cleanse in acids, the preceding brush with smaller and stiffer bristles is used (Fig. 178).

The gilder of watch-works has an oval brush (Fig. 177) with stiff and short bristles for graining the silver.

Fig. 176.

Fig. 177.

Fig. 178.

The galvanoplastic operator, for coating moulds with plumbago, besides a number of pencils, uses also three kinds of brushes-the watchmaker's (Fig. 178), a hatbrush, and a blacking-brush.

The bronzer uses all kinds of brushes.

Brushes are perfectly deprived of adherent grease by washing with benzole or bisulphide of carbon.

Burnishing Tools.

They have already been described in this work.

Evaporating Dishes or Capsules.

These are usually vessels of porcelain, and are intended to bear a high temperature. The best are thin and uniformly so. Like glass flasks, they should be supported

Fig. 179.

Fig. 180.

above the fire upon an iron stand and wire-gauze. As far as practicable they should be gradually heated and cooled. When taken from the fire, they should be placed upon rings made of plaited straw. They are