Page:Popular Science Monthly Volume 26.djvu/548

532 the proportion was too slight to be appreciable. Count Marsigli, who in the reign of Louis XIV tried to make sea-water artificially, took great pains to mix bitumen with the salts he put in solution, in order to make the reproduction perfect. The partisans of the theory cited the Dead Sea, which was near asphalt-beds, and the waters of which were insupportably bitter. But Macquer, assisted by Lavoisier, a hundred years ago, carefully distilled specimens of this water, and found in it no more bitumen than had been found in Mediterranean water—which was none at all. He attributed the bitter taste of this water to the presence of salts of magnesia.

It is not to-day that investigators have sought to make sea-water potable by removing its nauseous taste. The problem was solved long ago, and, as often happens, the usefulness of the invention once so greatly desired has been much depreciated. When fresh water for the provisioning of vessels was stored in wooden casks, it was liable to spoil in a short time. Now it is carried in large iron tanks, in which, instead of spoiling, it is improved by acquiring a ferruginous quality. The ancients did not venture far from the shores, and were contented with a simple coasting-trade; nevertheless, this question interested them, and Pliny describes two means of freshening the water of the Mediterranean, one of which is absurd and the other impracticable: One was to plunge into the sea hollow balls of wax, which, the author affirms, would be filled with pure water; and the other was to expose fleecy sheep-skins on the deck of the vessel, to collect the morning dews.

Whoever examines the series of memoirs published during the seventeenth and eighteenth centuries, on the subject of freshening seawater by distillation, must be struck by the divergence of opinions and the want of concordance in the results, some declaring that distilled water is pure, healthy, and tasteless, others that it is unhealthy and almost as detestable as before the operation. The differences between them are easily explained. Marine salt is not the only substance dissolved in the water, but is accompanied by several other bodies, the principal of which is chloride of magnesium. This salt when dry resists the action of the most violent heat without changing; but in boiling water undergoes a double decomposition, in which the chlorine leaves the magnesium to unite with the hydrogen of the water, while the oxygen thereof unites with the magnesium. There is thereby produced magnesia, which remains in the vessel, and hydrochloric acid, which is distilled over. Now, distilled water is made impotable and unhealthy by any traces of that acid. The difficulty may be obviated by previously removing such salts as can be made to settle, or by adding fresh sea-water. Water boils at a temperature several degrees higher than usual when it is charged with salts. If it is sufficiently diluted it will not disengage hydrochloric acid. Or the acid may be absorbed by substances added to the water for that purpose, and which