page 233 note * There is sometimes obtained in the large way, from the products of the evaporation of sea-water, a triple salt, which has not been noticed by chemists, but which appears to be of definite composition, and is distinguished by peculiar properties,—a Sulphate of Magnesia and Soda. It is formed in purifying the sulphate of magnesia procured by the first evaporation from the bittern of sea-salt. In this process the sulphate, which is impure, both from the intermixture of muriate of soda and muriate of magnesia, and perhaps, also, of sulphate of soda, is dissolved in water, and by evaporation and cooling is obtained crystallised; a fresh quantity of it is added to the residual liquor, and by the necessary evaporation and cooling, a new crystallization is produced; this is repeated for a third or fourth time; and it is in these latter crystallizations that this triple salt is formed, frequently in considerable quantity, and usually at a high temperature, being precipitated even in the boiler. It crystallises in rhombs, at first irregular and semitransparent; but by solution in water, and a second crystallization, is obtained in more regular rhombs, truncated on the acute angles, on the obtuse angles and edges, and on the terminal edges, considering the rhomb as a four-sided prism, and transparent. The crystals are permanent in the air; they are soluble in little more than three times their weight of water, at the temperature of 60° they do not undergo the watery fusion from heat, but suffer decrepitation. In these properties, this salt differs entirely from sulphate of soda, or sulphate of magnesia.
To determine its composition, 20 grains reduced to powder were exposed to heat, raised gradually nearly to redness; they lost from the escape of water 5.6 grains. The residual powder was dissolved in water, and muriate of barytes was added as long as any precipitation was produced. The precipitate dried at a red heat, weighed 23.9 grains, equivalent to sulphuric acid 8.2 grains. To the clear liquor carbonate of ammonia was added, which did not impair the transparency; phosphoric acid was then dropped in, which produced a copious precipitation. The precipitate, calcined at a red-heat, weighed 5.3 grains, equivalent to 2.1 of magnesia, or 6.4 of sulphate of magnesia; the residual liquor being evaporated to dryness, the dry mass was submitted to heat, gradually raised, as long as any vapours exhaled; it afforded, by solution in water and evaporation, muriate of soda in cubes, which, after exposure to a red heat, weighed 6.4 grains, equivalent to 7.8 grains of sulphate of soda. 100 grains of the salt, therefore, afford of
It afforded also a slight trace of muriatic acid; its solution being in a very slight degree rendered turbid by nitrate of silver, probably owing to the intermixture of a little muriate of soda, as an extraneous ingredient. This accounts for the proportion of sulphuric acid, as inferred from the quantities of the bases, being a little larger than that directly obtained by the precipitation by muriate of barytes.
The difference of crystalline form, as well as other differences of properties in the salt from those, either of sulphate of soda or sulphate of magnesia, sufficiently prove that it is not merely an intermixture of the two, but that it is of definite composition. It deserves to be remarked, too, that it has not the same relation to water that either of these salts has, or any mean between them; the quantity of its water of crystallization being considerably less. Its taste is much less disagreeable than that of sulphate of soda or sulphate of magnesia; it might therefore probably be introduced with advantage as a purgative salt, especially as it could be procured at a low price; and from its composition, it would afford a very good substitute for the aperient mineral waters, which usually owe their activity to sulphate of soda and sulphate of magnesia.
