It is evident that it will be possible to vary ad infinitum the mixtures and the proportions of the component metals, and thus to arrive at an unlimited number of alloys, which on account of slight differences of color, ductility, sonorousness, etc., have received a great variety of names.*

Amalgams

Are alloys of one or several metals with mercury, but the latter must absolutely be one of the component parts.

Only two amalgams are of importance in our art that of Darcet, already described; and that of gold, which is the basis of fire-gilding.

Whatever be the proportions of gold and mercury put together, an amalgam is always formed; but there are certain proportions which are more or less favorable for obtaining a certain result. Thus, the gold amalgam for gilding by stirring should be more fluid, and therefore contain more mercury, than that prepared for dead gilding by fire, or for ormolu. The latter should be of the consistency of cold butter, a little rough to the touch, and with a crystalline texture, which causes the production of a crackling noise when the amalgam is pressed between the fingers. That for gilding by stirring should be of the consistency of honey and quite soft, since in this condition it easily spreads itself over the surface of a multitude of small articles.

An amalgam is generally prepared by heating distilled mercury to a temperature of about 400° Fah., and adding to it gold in the shape of a leaf or ribbon, which becomes readily incorporated with it. The whole is then thrown. into cold water. If the proportion of mercury has been

*For a full description of alloys and amalgams, see "A Practical Guide for the Manufacture of Metallic Alloys," from the French of A. Guettier, by A. A. Fesquet. Philadelphia: H. C. Baird, 1872.

too great, the amalgam may be heated over the fire, until by the volatilization of part of the mercury, the proper consistency is reached. On the other hand, should the amalgam be too hard, it may be softened by the addition of mercury, which soon mingles with the mass.

When such an amalgam is heated to a dull red heat, all of the mercury is volatilized, and the gold remains in the form of a spongy and crumbling mass. Gold, in the presence of mercury salts, does not form amalgams like copper; it is absolutely necessary that the two metals should be in the metallic state in order that they shall amalgamate.

Mercury is often adulterated by the addition of bismuth, lead, or tin; and it is evident that these various metals, which do not volatilize, remain with the gold and deteriorate it.

Ammonia.

(Spirit of hartshorn. Volatile alkali, etc.)

This compound, which generally bears the name of ammoniacal gas when in the gaseous form, and of ammonia when in solution, presents properties similar to those of potassa, soda, and other alkalies which are metallic oxides. On account of this analogy, therefore, it is customary to consider it as the oxide of a hypothetical metal, ammonium, a compound radical composed of nitrogen and hydrogen. This hypothesis, whether correct or not, has the advantage of convenience.

Gaseous ammonia dissolves in water eagerly, one vol ume absorbing, when cold, about 500 volumes of the gas. Notwithstanding this remarkable solubility, ammoniacal gas does not produce visible fumes even in contact with very moist air, as is the case with hydrochloric acid and other substances.

Aqua ammonia is a colorless liquid, possessing a characteristic and overpowering pungent smell.

Ammonia restores the blue color of litmus reddened by an acid, and saturates the affinities of the most powerful acids; and, on this account, is often employed for removing acid stains upon clothes. We should except, however, the stains produced by nitric acid, which become more visible instead of disappearing. All ammoniacal

salts, except the carbonates, are odorless.

Ammonia is obtained, even in the cold, by treating any ammoniacal salt with a fixed alkali. Sulphate or chloride of ammonium is generally employed for this purpose, and is generally heated in a suitable retort with slaked lime. The gas produced is collected by displacement of mercury under bell glasses, or in tubes of glass, where it is wanted in the gaseous state; or is dissolved in the water contained in the flasks of a Woulf's apparatus (Fig. 168),

if we desire its solution. After the operation, there remains in the retort the sulphate or chloride of calcium according to the nature of the ammoniacal salt employed.

By far the greater proportion of the commercial aqua ammonia is, at the present time, obtained from the ammoniacal liquors of coal-gas works.

Ammonia is employed for aiding the solution of the copper salts entering into the composition of the baths