solution of KOH or NaOH is obtained which is sold in this state, or “finished” as solid caustic in the manner described in the section treating of the Leblanc soda.
(2) The Castner-Kellner process employs no diaphragm, but a mercurial cathode. The electrolysis takes place in the central compartment of a tripartite trough which can be made to rock slightly either to one side or the other. The bottom of the trough is covered with mercury. The sodium as it is formed at the cathode at once dissolves in the mercury which protects it against the action of the water as long as the percentage of sodium in the mercury does not exceed, say, 0·02%. When this percentage has been reached, the cell is rocked to the other side, so that the amalgam flows into one of the outer compartments where the sodium is converted by water into sodium hydrate. At the same time fresh mercury, from which the sodium had been previously extracted, flows from the other outside compartment into the central one. After a certain time the whole is rocked towards the other side, and the process is continued until the outer compartments contain a strong solution of caustic soda, free from chloride and hypochlorite.
(3) Aussig process.—Here the anode is fixed in a bell, mounted in a larger iron tank where the cathodes are placed. The whole is filled with a solution of common salt. As the electrolysis goes on, NaOH is formed at the cathodes and remains at the bottom. The intermediate layer of the salt solution, floating over the caustic soda solution, plays the part of a diaphragm, by preventing the chlorine evolved in the bell from acting on the sodium hydrate formed outside, and this solution offers much less resistance to the electric current than the ordinary diaphragms. This process therefore consumes less power than most others.
(4) The Acker-Douglas process electrolyses sodium chloride in the molten state, employing a cathode consisting of molten lead. The latter dissolves the sodium as it is formed and carries it to an outer compartment where by the action of water the sodium is converted into caustic soda, while the lead returns to the inner compartment. This process is carried on at Niagara Falls, but it is uncertain to what extent.
(5) The Hargreaves-Bird process avoids certain drawbacks attached to other processes, by employing a wire diaphragm and converting the caustic soda as it issues on the other side of this, by means of carbon dioxide, into a mixture of sodium carbonate and bicarbonate, which separates out in the solid state. This process is but little used.
It stands to reason that the electrolytic processes have been principally developed in localities where the electric current can be produced in the cheapest possible manner by means of water power, but this is not the only condition to be considered, as the question of freight to a centre of consumption and other circumstances may also play an important part. Where coal is very cheap indeed and the other conditions are favourable, it is possible to establish such an industry with a prospect of commercial success, even when the electric current is produced by means of steam-engines.
Natural Soda.—This is the term applied to certain deposits of alkaline salts, or their solutions, which occur, sometimes in very large quantities, in various parts of the world. The oldest and best known of these are the Natron lakes in Lower Egypt. The largest occurrence of natural soda hitherto known is that in Owen’s Lake and other salt lakes situated in eastern California. The soda in all of these is present as “sesquicarbonate,” in reality 4′3 carbonate: NaHCO3·Na2CO3·2H2O, and is always mixed with large quantities of chloride and sulphate, which makes its extraction more difficult than would appear from the outset. Hence, although for many centuries (up to Leblanc’s invention) hardly any soda was available except from this source, and although we now know that millions of tons of it exist, especially in the west of the United States, there is as yet very little of it practically employed, and that only locally.
References.—The principal work on the manufacture of alkali is G. Lunge’s Sulphuric Acid and Alkali (2nd ed., vols. ii. and iii., 1895–1896). This work has also appeared in a German and a French edition. The same author wrote the articles on the manufacture of sodium and potassium compounds and on chlorine in Thorpe’s Dictionary of Applied Chemistry (3 vols., 1890–1893). The subject is also treated, very much more briefly, in Sorel’s Industrie chimique minerale (1902), and of course in every other general treatise on chemical technology. A special treatise on the manufacture of ammonia soda ash has been published in German by H. Schreib. Consult also the official Annual reports on Alkali, &c., and, from 1864 onwards, Journal of the Society of Chemical Industry, Fischers Jahresberichte der chemischen Technologie, and Zeitschrift für angewandte Chemie. (G. L.)
ALKALINE EARTHS. The so-called alkaline earth-metals are the elements beryllium, magnesium, calcium, strontium and barium. By the early chemists, the term earth was used to denote those non-metallic substances which were insoluble in water and were unaffected by strong heating; and as some of these substances (e.g. lime) were found to be very similar in properties to those of the alkalis, they were called alkaline earths. The alkaline earths were assumed to be elements until 1807, when Sir H. Davy showed that they were oxides of various metals. The metals comprising this group are never found in] the uncombined condition, but occur most often in the form of carbonates and sulphates; they form oxides of the type RO, and in the case of calcium, strontium and barium, of the type RO2. The oxides of type RO are soluble in water, the solution possessing a strongly alkaline reaction and rapidly absorbing carbon dioxide on exposure; they are basic in character and dissolve readily in acids with the formation of the corresponding salts. As the atomic weight of the element increases, it is found that the solubility of the sulphates in water decreases.
Beryllium to a certain extent stands alone in many of its chemical properties, resembling to some extent the metal aluminium. Beryllium and magnesium are permanent in dry air; calcium, strontium and barium, however, oxidize rapidly on exposure. The salts of all the metals of this group usually crystallize well, the chlorides and nitrates dissolve readily in water, whilst the carbonates, phosphates and sulphates are either very sparingly soluble or are insoluble in water.
ALKALOID, in chemistry, a term originally applied to any organic base, i.e. a nitrogenous substance which forms salts with acids; now, however, it is usual to restrict the term to bases of vegetable origin and characterized by remarkable toxicological effects. Such bases occur almost exclusively in the dicotyledons, generally in combination with malic, citric, tartaric or similar plant-acids. They may be extracted by exhausting the plant-tissues with a dilute acid, and precipitating the bases with potash, soda, lime or magnesia. The separation of the mixed bases so obtained is effected by repeated fractional crystallization, or by taking advantage of certain properties of the constituents.
A chemical classification of alkaloids is difficult on account of their complex constitution. I. A. Wyschnegradsky, and afterwards W. Königs, expressed the opinion that the alkaloids were derivatives of pyridine or quinoline. This view has been fairly well supported by later discoveries; but, in addition to pyridine and quinoline nuclei, alkaloids derived from isoquinoline are known. The purely chemical literature on the alkaloids is especially voluminous; and from the assiduity with which the constitutions of these substances have been and are still being attacked, we may conclude that their synthesis is but a question of time. Piperine, conine, atropine, belladonine, cocaine, hyoscyamine and nicotine have been already synthesized; the constitution of several others requires confirmation, while there remain many important alkaloids—quinine, morphine, strychnine, &c.—whose constitution remains unknown.
The following classification is simple and convenient; the list of alkaloids makes no pretence at being exhaustive.
- (1) Pyridine group. Piperine; conine, trigonelline; arecaidine; guvacine; pilocarpine; cytisine; nicotine; sparteine.
- (2) Tropine group. Alkaloids characterized by containing the tropine (q.v.) nucleus. Atropine; cocaine; hygrine; ecgonine; pelletierine.
- (3) Quinoline group. The alkaloids of the quina-barks:
