Filtration, in chemistry, as well as in domestic economy, is the process of straining or filtering liquors by means of woollen cloth, cotton, linen, paper, or other mate-rials. - It deserves to be previously remarked, that in every attempt at purifying fluids in the manner here, alluded to, we can divest them only of those foreign ingredients which are mixed with them, and not of such as they hold in solution. The former fit

Former may be separated from them, by proper filtration; but the latter must be disenraged, either by pre-cipitation or distillation. Although the utility of filtration is thus limited to the noxious particles mixed with liquid bodies, such as foul water, yet it is sufficiently important to deserve some attention.

The common niters are of two sorts; namely, simple pieces of paper, or cloth, through which the fluid is passel; or similar materials twisted up in the same manner as skeins or wicks ; they are first wetted, then squeezed. and one end put into the vessel, which contains the liquor to be filtrated; the other end is to be suspended beneath the surface of the liquor, the purest parts of which drop gradually out of the vessel, leaving behind the coarser particles.

These filters, however, are not calculated for domestic use : hence different machines have been invented for the purpose of purifying turbid water. But among these various contrivances, few appear to possess the advantage of simplicity, combined with that of affording an ample supply of a fluid so essentially necessary to the pre-servation of health. - A patent has lately been granted to Mr. James Peacock, of Finsbury-square, for a filtering machine, which is stated to be superior to any hitherto invented. It completely accomlishes the purpose of filtration, by causing the turbid fluid to ascend through a medium of fine gravel, of progressive degrees of fineness, by which means the foulest water or other fluid becomes perfectly freed from all (mixed) impurities, without any noxious mineral quality, which pumice or other common filtering stones are suspeded to communicate. Should, from continual use, its operation become in any degree impeded, it may be completely cleansed with the greatest facility in the short space of one minute: an advantage possessed by none of the common ma- " chines that operate by descent. Beside these useful properties, Mr. Peacock's filtering machine does* not occupy more room than a large drip-stone with its apparatus, and yields a constant and pure stream of more than 300 galons in 24 hours. - A specimen of this machine is deposited for inspection at Guildhall, London.

As we are, from a principle of justice to the public, no advocates for patent inventions that upon the whole, arise from the same mercenary and contracted source as patent quack medicines; we shall recommend a very simple and effectual apparatus, by which the purest water may be easily pro-curd. This contrivance is calculated on the plan of the celebrated filtering machine erected at Paris, in the vicinity of the Samamtoine, and by means of which the foul water of the river Seine is so completely purified, as to be divested of its laxative properties. Besides, this machinery, if constructed on a large scale, is well adapted to supply the largest breweries, or dye-ing works, with any quantity of pare water at a trilling expente, and is attended with very little ad-ditional trouble.

When we reflect on the method which Nature pursues in the filtration of water, we rind that such waters as descend from hills, tho' passing through sand and rocks, are seldom perfectly pure ; but that those are the most limpid, which, by ascending, ooze out near the foot of a mountain. The cause of this difference appears to be owing to the circumstance, that if the water only descends through sand, the finest and most weighty foreign particles gradually penetrate through the sandy strata ; on the contrary, when it is forced to rise through sand, all such ponderous ingredients settle at the bottom ; because, from their greater specific gravity, they cannot ascend to the top. The lighter particles'of fluids, consequently, in both cases remain in the upper strata of the earth or sand.

From these considerations, Professor Parrot, jun. of Paris, was induced to give his filtering machine the form represented in the following cut:

filtering machine

The principal part of the machinery consists of a square vessel, bent in the form of an inverted syphon. The curve may be circular, elliptic, or in any other direction. This vessel is filled with fine, pure sand, till nearly the height of the dotted line x, y, which denotes the ascent of the water to D, whence it flows into the receiver. The part marked A, B, should always project above this line, according to the size of the filtering machine. To A, B, there is attached a woollen bag, which is open at the top, and the lower part of which touches the sand. It serves the purpose of collecting the coarsest impurities, and thus preserves the sand for a longer time from becoming foul. The bag, therefore, may occasionally be removed, and rinsed in clean water. - It is evident, that the water flows at A, through the bag into the fill re, and rises at the place marked D, which is considerably lower than the former. It affords a very agreeable sight to Observe the most limpid fluid penetrating the uppermost stratum of sand, perfectly similar to that oozing from the purest natural spring.

Prof. Parrot remarks that he procured a filtering machine made of block-tin, for ascertaining by experiments the purity and quality of water, that may thus be obtained in a given time. It consisted of the following dimensions: the small diameter B,E, was eight Paris inches,; the large of the whole machine, eleven inches; consequently the thickness of the vessel A, B, was one inch and a half ; - the breadth of it, two inches and seven-eighths. The perpendicular height of the lower side, from C, its basis, to the rim D, whence the water issues, was four inches and one-twelfth - the opposite height of the mouth A,B, eight inches and three-fourths 5 and the' height of the sand on the side marked D, was three inches und-one-sixth.

Although, in experiments of this nature, much depends on the relative size and purity of the sand, which necessarily afford different results, yet Prof. Parrot has, after repeated trials, deduced thefollowing conclusions, which appear to be well-founded.

1. That the difference of the niveau, or water-level, as an essential influence on the quantity of the purified water thus obtained ; 2. That a prolongation of the stratum of sand does not considerably diminish the product of the filtre, but remarkably contributes to the purity of the fluid. 3. That if the water be forced to pass through the sand with increased velocity, it will be less pure than by allowing it a proper time for its passage; and, 4. That a machine of the dimensions above described, will furnish about three quarts of water in an hour, or eighteen gallons in twenty-four hours. This quantity, however, being too large in proportion to the size of the machine, it is advisable, either to lessen the difference of the water-fall; or, which is still better, to prolong the stratum of sand, in order to reduce the nitration of the water to half the quantity above stated, and to obtain it in greater purity. Thus, a filtering apparatus eighteen inches long from AtoD, two inches thick, and four broad, would afford every hour six pints of very pure water. If, therefore, so small a machine, containing a very moderate stratum of sand, and requiring only a difference of two or three inches in the height of the water, furnishes a clear and pure fluid, it follows that an apparatus on a larger scale, provided with a bed of sand from five to six feet long, and admitting of a difference from twelve to eighteen inches in the fall of the water, might be usefully employed in public wells,hydraulic machines, and even in camps, for the supply of an ar my.

In the construction of large filtering machines, Prof. Parrot justly observes, that they should not be extended in the direction A,C,D, to a greater length than is absolutely necessary; as, in this case, they will not require any considerable difference in the fall and rise of the water: on the other hand, their breadth and thickness may be accordingly increased. - • Thus, the diameter of such a machine would still more resemble that of a syphon, as is represented in the annexed cut.

large filtering machines

This form might also be adopted for smaller machines, especially such as are designed for travellers, two of whom might be amply provided with pure water, and in a very short time, by a vessel of the following dimensions: fromP, toQ, eight inches long; from P, to R, twelve inches high; and the whole four inches in breadth.

If the form last delineated be employed on an extensive scale, there should be a trap door in the lowermost part marked R, so constructed that it may fit exactly, and admit no passage to the water : this aperture would serve only for the removal of the sand, when it is ren-dered foul by long use. In the smaller machines, intended for travelling, such a door is unnecessary, as they may be easily emptied of their contents through either of the orifices P, or Q. Instead of tins addition to the latter, the upper room (which in the first of these cuts is circumscribed with the letters B,F, E.), might serve as a reservoir of pur- water, that could either be decanted, or drawn off by means of a cock applied to the centre of the machine, marked F. We think, however, this latter arrangement, which is proposed by M. Parrot, in many respects objectionable, and therefore advise the reader to make use of the more simplified construc-tion. Hence we shall only add, that every filtering machine ought to be provided with a cloth cover, to prevent the dust from rising with the water, without impeding its filtration.

It is needless to expatiate on the great advantages of filtering machines in the different processes of dyeing, baking, brewing, distilling, and all the domestic arts. As no particle of real nutriment can be assimilated to the human fluids, without being previously macerated and reduced by water (whether this fluid be introduced into the stomach, in the form of beer, wine, spirits, tea, etc.) it will be easily understood that impure water cannot fail to produce, however slowly, many dangerous, and often incurable diseases - the source of which is seldom suspected. - See Water.