When an infusion of litmus, which has been reddened by a few drops of an acid solution, is mixed with a slight excess of an alkali, the colour of the infusion changes to purple or blue; by applying an excess of acid to the same infusion, the original red colour is restored. In like manner the purple infusion obtained by macerating red cabbage in hot water, and also the infusion of the petals of the red rose, become green by the addition of an alkali, and crimson by that of an acid. Turmeric and many other yellow dyes are rendered brown by an alkali, but the original yellow is restored on applying an acid. Alkaline carbonates produce the same effects on vegetable colouring matters as the caustic or pure alkalies.

A class of bodies distinguished as alkaline earths, to which belong lime and barytes, act on such colouring matters in precisely the same manner as alkalies, which they also resemble in some other respects. The distinction between an alkali and an alkaline earth is founded merely on the solubility or insolubility of the carbonate of the base in water; the carbonate of an alkali proper being soluble, and that of an alkaline earth insoluble.*

§ 1. SODIUM, CAUSTIC SODA, AND THE CARBONATES OF

SODA.

Sodium, the metallic basis of soda, was first obtained by Sir H. Davy, in 1807, by subjecting moist

In like manner, the distinction between an alkaline earth and an earth proper, as alumina, is founded on the solubility or insolubility of

ened hydrate of soda to voltaic decomposition, when the oxygen both of the water and the soda were liberated at the positive pole, and the sodium of the latter at the negative pole. A more advantageous method of procuring this metal has since been devised, which consists in strongly heating in an iron distillatory apparatus an intimate mixture of carbonate of soda and charcoal. The oxygen of the soda unites with the charcoal; while the liberated sodium, being volatile at a red heat, rises in vapour and is condensed in the receiver, which should contain rectified naphtha or petroleum to preserve the sodium from oxidation by the air.*

Sodium has considerable lustre, and in colour is very similar to silver. Its specific gravity, according to Gay-Lussac and Thénard, is 0.972 at 59°. At common temperatures, it is sufficiently soft to be reduced to thin leaves by the pressure of the fingers. It is liquid at 200°, but does not rise freely in vapour under a bright red heat. It presently becomes tarnished on exposure to the air, owing to the formation of a crust of soda; and takes fire and emits

the base itself; alkaline earths are more or less soluble, but earths proper are quite insoluble.

*For a description of the proper apparatus for preparing the metallic bases of the fixed alkalies, see Graham's "Elements of Chemistry," P. 440.

The following mode of obtaining sodium is recommended by M. Schoedler. Three pounds of commercial acetate of soda are ignited, the free access of air being avoided; and the residue of carbonate of soda and charcoal, which weighs about one pound, is mixed with a quarter of a pound of finely powdered charcoal and half a pound of charcoal in a coarser state, to prevent fusion of the mass. The mixture, when heated in the usual manner in one of the iron bottles in which mercury is imported, should yield between four and five ounces of pure sodium.

brilliant scintillations when heated nearly to redness. When thrown on water, it oxidates with great vivacity; hydrogen gas is disengaged, and an alkaline solution of soda formed: on cold water it does not inflame as potassium does, but on hot water it scintillates and sometimes takes fire. If moistened with only a few drops of cold water, it becomes sufficiently hot to inflame.

The commonly received equivalent of sodium is 23.3. Its symbol is Na (from natrium). Sodium forms at least two compounds with oxygen; one of which, the basis of common soda, contains one equivalent of metal to one of oxygen; the other oxide is a peroxide, possessing neither acid nor alkaline properties, composed of two equivalents of sodium to three equivalents of oxygen. The peroxide is decomposed by contact with water into soda and oxygen gas.

Soda.-Pure soda, free from water, can only be obtained by the combustion of pure sodium in dry air or oxygen gas. The solid residue of the evaporation of a solution of caustic soda (soda ley), retains one equivalent of water to one equivalent of soda, even when heated to redness.

Anhydrous soda is a grey solid, of difficult fusibility, having a very powerful affinity for water. The hydrate is white and brittle, very caustic, possessed of powerful alkaline properties, easily fused by heat, and readily dissolved by water and alcohol. Its specific gravity is 1.536. At a very elevated temperature it is dissipated as a vapour. A solution of caustic soda dissolves wool, hair, the skin, and most

animal matters; tallow and other unctuous matters, with the formation of soaps; sulphur and some metallic sulphurets; silica, alumina, and several metallic oxides quite insoluble in water.

Solution of caustic soda is always prepared by decomposing carbonate of soda by means of caustic lime in the presence of water. The lime then unites with

the carbonic acid of the alkaline carbonate to form carbonate of lime, and alkali is left in a state of causticity.*

In preparing soda leys on an extensive scale, where absolute purity is not essential, as in the soap manufacture, the mixture of lime and carbonate of soda or soda-ash is placed in large cast-iron vats and covered with water, and after some hours, the ley is withdrawn through a plug at the bottom of the vat, and either fresh water or a weak ley introduced at top. (See the description of the mode of preparing soda leys for the soap manufacture in the article on Soap, p. 300.)

A much purer ley may be prepared by boiling in a clean iron pot a solution of two parts of crystallized carbonate of soda in eight or ten parts of water, with about one part of hydrate of lime, until a little of the liquid from which the insoluble carbonate of lime has been separated by subsidence and decantation ceases to effervesce when saturated by an acid. The pot may then be removed from the fire, carefully covered to prevent the ley from absorbing carbonic acid from the air; and when the insoluble carbonate of lime and

* The power of lime to decompose alkaline carbonates is perceived on mixing clear lime-water with a clear solution of carbonate of soda or potash, when an immediate precipitation of carbonate of lime appears.

the excess of hydrate of lime have subsided, the clear ley may be decanted into stoppered vessels for use, or be evaporated in an iron or silver vessel until it assumes an oily appearance, and then be poured upon an iron or stone slab or into iron moulds to solidify. The access of air should be avoided as much as possible during the concentration of the ley.

The solution of soda made as above from soda-ash is contaminated with common salt and sulphate of soda, both of which may be separated by concentrating the solution considerably, these salts being insoluble in a strong solution of soda.

The following table, founded on the experiments of Richter, exhibits the proportion of hydrate of soda and of dry caustic soda contained in leys at different densities:

Carbonate of soda.-The common mode of manufacturing this useful salt for commercial purposes forms the subject of another section of the present article. Its chemical history may be conveniently considered here.