Page:Popular Science Monthly Volume 6.djvu/457

Rh and, after a time, runs out below, highly charged with the soluble salt of the ashes.

But, although this machine has been so long in use, the principle on which it acts does not seem to have been fully understood until quite lately. About the year 1833 Messrs. Boullay, of Paris, applied the same apparatus to the manufacture of pharmaceutical preparations, tinctures, infusions, extracts, etc., and it at once became popular with pharmaceutists, under the name of Boullays' Filter, or the Displacement Apparatus. Its mode of operation is simple: the first portion of liquid poured in sinks into the powder that is to be exhausted, and saturates itself with the soluble parts of it. The later additions of liquid, instead of mixing with the first, drive it down before them and take its place, to yield it in turn to the next portions poured in. Thus the first portions of liquid that run from the bottom of the filter will, if it has been properly managed, contain nearly all the soluble matter, and the last will be almost unchanged. For example, if an ounce of powdered ginger be put into a glass tube, as a small lamp-chimney, over the lower end of which a piece of cotton cloth has been tied, and alcohol be slowly poured through it, the first fluid-ounce (f ℥ j) that comes through will contain about all the strength of the ginger. Looking through the glass, we can watch the whole process, see the first alcohol dissolve the resinous matter of the ginger, becoming thick and dark-colored in consequence, and then falling down before the new colorless alcohol added above.

Applying this principle of displacement to a sand-island, we have to start with a great heap of sand with its level or hollow top just above the reach of the waves, while the great mass of sand is below the sea-level. Of course all the submerged part is full of salt-water, while capillary attraction carries some of the water above the sea-level. The first rain that falls sinks at once into the loose sand. It rains very heavily in the gulf islands, but I never saw water run off of the surface; it sinks at once, and, in sinking, drives the salt-water before it, "displaces" it as the apothecaries say, but does not mix with it. Repeated rains continue the operation until there is a lake of fresh water held by the sand in the midst of the sea.

Fine sand will hold, between its grains, over one-third of its bulk of water; thus, within twelve feet of the sea-level, we have a lake of fresh water four feet deep, or about 1,250,000 gallons to each acre of island, and this is the supply reached by digging. But, should a long drought occur, evaporation would rapidly reduce the supply, and the sea-water following in will take its old place. Hence it was that, in 1864, we had plenty of good water at Ship Island and Santa Rosa, while at Brazos Santiago, where no rain had fallen, at the time of my visit, for ten months, the water was brackish and unfit for use, and the government was obliged to distill water from the sea for the use of the army, and the only vegetation on the island appeared to be