The engraving on the next page represents ao apparatus contrived by Messrs. Williams and Doyle, for the purpose of separating the salt from sea-water, by merely causing it to percolate through a body of sand under mechanical com pression, and thus to render it fresh. Could this object be obtained by such means, the invention would doubtless be one of the utmost importance to navigation, as it would render a store of fresh water unnecessary, thereby affording additional stowage for provisions or cargo ; but we are not aware of any experiments proving that substances dissolved, and chemically combined with a liquid, can be separated by filtration ; we therefore apprehend that the apparatus would be ineffectual for the object the inventors had in view, although it may prove very efficient in freeing water from any impurities floating or suspended in it. The following description of the engraving (which represents one of the several modes of construction proposed by the inventors), is derived from the specifi cation of their patent. a is a part of the cask supposed to contain sea water; a tube descendinglherefrom, made fast by bands c c c to the filtering appa ratus d 4 which is a strong square trunk of wood, lined internally with sheet lead, which are cemented together to prevent the interposition of water. This part of the apparatus is given in section, that the construction and arrange ment may be seen at one view ; e is the lower chamber, where the water is firsCreceived; f is a strong stool of open frame work, supported on five stout legs, g. A plan of this stool is given in a separate figure F, the situation of each of the five legs being marked with a g. Over this short frame is nailed a plate of copper, pierced with numerous small holes ; this plate is also shown by a separate figure H. Over the perforated plate are several layers of woollen cloth, or woven horse-hair i, and above these a body of sand k, filling up the entire trunk ; on the top is placed a sliding cover 1, which is operated upon by a strong screw m, working through a fixed nut n, which is supported by curved iron arms, extending from opposite sides of the trunk. The sand having been compressed, by the agency of the screw, into a more dense and compact mass, is prevented from rising by the pressure of the water, which percolating through the minute interstices to regain its level, deposits its salt, and runs out by the pipe o in a fresh state into a vessel p placed to receive it. When the sand has become saturated with salt, it is to be removed by taking out the screw and the press ing board 1; the man holes r r may then be opened by unscrewing the plugs, when the other materials may be easily sifted. These matters being completed,
a fresh quantity of sand may be taken from the ballast of the ship, and the process of filtration continued as before.
We shall dose this article with a description of a very convenient apparatus for filtering liquids out of contact with the atmosphere, invented by Mr. Donovan. By means of this arrangement, alkalies can be preserved in their caustic state, the absorption of carbonic acid by the alkali bemg prevented. a is a bottle of green glass, with a funnel-shaped end inserted into another bottle b, the junction being luted or ground to fit closely ; the neck d of the upper vessel has a cork tightly fitted to it, perforated in the middle for the reception of the glass tube c, which being bent down wards, enters the branched neck e of the lower vessel, thus connecting them together, and opening an air passage between them. The funnel-shaped end of the upper vessel has a piece of linen, loosely rolled up, placed in it, for the purpose of filtering, but for the corrosive acids a stratum of pounded flints should be employed I instead of the cloth. To charge the upper vessel with the alkaline solution, the tube c must of course be removed, and the first droppings should be allowed to run to waste previously to the apparatus being fitted together, that no absorption of carbonic acid may take place in the filtered liquor. When the whole is properly closed, the filtration will proceed without the possibility of absorption. Now it is evident that no liquor can fall from the upper vessel without an equal volume of air entering it, and that none can enter the lower without an equal bulk escaping from it. Both these conditions are fulfilled by the connecting tube c, the air being driven from the lower into the upper vessel at every dropping of the filtered liquid. The whole process is therefore conducted without the access of more air than the vessels at first contained, and in the most cleanly and perfect manner. The utility of this contrivance is very extensive.
The most volatile liquids, as ether, alcohol, ammoniacal liquors, volatile oils, &c. may be filtered without loss, as the vapours cannot escape during the operation ; and by the exclusion of the atmosphere in the filtration of a variety of fluids, other injurious effects to which they are subject by the common process, may be entirely obviated.