and 86° Fahr. (25° and 30° Cent.), the crystallization may be considered complete. The mother-liquor is then rapidly withdrawn by means of a large siphon, and the liquid which remains between the summits of the crystals imbibed by a sponge. The cover of the vat is replaced, and the crystalline mass is allowed to stand for five or six hours, so that its temperature may be gradually reduced to that of the external air. The crystals are then detached and assorted for the market. To procure borax in the octohedral form (see page 264), the solution of the crude salt should be made of the specific gravity 1.261 (30° Baumé), at the temperature of 212°, before being set to crystallize. The deposition of octohedral borax commences when the temperature of the solution falls to 174° Fahr. (79° Cent.), and terminates at the temperature of 133° Fahr. (56° Cent.). When at the latter temperature, the mother-liquor should be instantly withdrawn by a siphon to prevent the crystallization of ordinary prismatic borax. The liquor gives an abundant deposit of the latter applicable to several processes in which the salt is used.

Another method of preparing borax from the native boracic acid is by calcining a mixture of the latter (after being washed with water) with nitrate of soda. This process may be conducted very economically, provided the nitrous acid disengaged as gas through the decomposition of the nitric acid of the nitrate of soda be made available in the preparation of oxalic acid from sugar, or some other important manufacturing process. But it is difficult to find a material well adapted for the vessel to contain the

mixture, as siliceous substances are acted on by fused soda and borax with great facility; and an iron vessel would become rapidly corroded by nitrous acid.

The most economical process for preparing borax from boracic acid is the following, for which a patent has been recently obtained by Mr. Charles M. Sautter (May 1843). About 38 parts by weight of pure crystallized boracic acid (3HO+ BO,, or else a corresponding quantity of the washed native acid) are finely sifted and intimately mixed with about 45 parts by weight of powdered crystallized carbonate of soda (NaO, CO, +10HO), and the mixture is placed upon wooden planks in layers of about one inch thick, in a room heated to any temperature between 90° and 115° Fahr. In the course of from twenty-four to thirty-six hours, the hours, the mass being occasionally stirred, the whole of the boracic acid and soda will have combined to form borax, the carbonic acid and superabundant water being expelled. The product thus obtained is fully adapted to all the applications of ordinary borax; but being in a granular state and not perfectly white, it will not, in all probability, meet with an extensive sale, at least for a few years hence.

Borax is sometimes sophisticated with common salt, and occasionally with alum. The presence of the former is easily detected by adding a few drops of a solution of nitrate of silver, which would afford a white curdy precipitate of chloride of silver, insoluble in nitric acid if the smallest quantity of common salt is present. The presence of alum in the borax may be detected by ammonia, which gives,

VOL. II.

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293

SOAP.

§ I. General principles of Soap-making Processes.-II. Materials employed in Soap-making.-III. Manufacture of Hard Soaps.-IV. Manufacture of Soft Soap.

SI. GENERAL PRINCIPLES OF SOAP-MAKING PROCESSES.

As a generic term, soap is sometimes used to denote all kinds of combinations of unctuous matters with bases; whether alkalies, as potash and soda; earths, as lime and magnesia; or oxides of metals proper, as oxide of lead and oxide of zinc. This extended meaning of the term is hardly recognized, however, in common language; a soap being usually considered a compound of an unctuous substance with either potash or soda.

Though made by mixing an oil or a fat with an alkali, soap is not to be considered as a direct combination of the oil or fat, as a whole, with the alkali. When liberated from its combination with the base, by applying a strong acid, the oil is found to be altered in properties, being more soluble in alcohol and more easily saponified than before, and to have experienced an alteration in composition. This substance is capable of combining as a whole with a base to form a soap; but the original oil or fat contains another constituent besides the oil or fat сараble of being eliminated from the soap by a strong acid.

In the course of an extended chemical examination of fixed oils and soaps, which occupied about fourteen years, M. Chevreul discovered that nearly all fixed oils and fats contain always two, and generally three, distinct compounds, united in the most various proportions. One of these, liquid at ordinary temperatures, has been named oleine; the other two, which are solid, but differ in fusibility, are named stearine and margarine. M. Chevreul also ascertained that each of these bodies contains an organic base,* soluble in water, named glycerine or oxide of glycerule, in combination with an unctuous substance, possessed of the essential characters of an acid. The acid constituent in each of the three fatty principles is peculiar, but the glycerine is common to them all. The acid in oleine is termed oleic acid; that in stearine, stearic acid; and that in margarine, margaric acid. Oleine, stearine, and margarine have therefore a saline constitution, and may be respectively regarded as the oleate, stearate, and margarate of glycerine.†

These fatty principles, when brought into contact with a free alkali, oxide of lead, or oxide of zinc, in the presence of water become decomposed; their acids leave the glycerine to unite with the alkali or metallic oxide (which are more powerful

By an organic base is meant a substance capable of uniting with acids to form, in most cases, a neutral compound, like potash, soda, and other metallic oxides; but not derived, as these are, from the mineral kingdom.

It is uncertain, however, whether the glycerine actually pre-exists as such in the oil, or is merely the product of the action of the base on the oil. The question is one of no practical importance whatever, since the oil is decomposable in this manner by very feeble bases.