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Hie magicus afi'ert cantus, hie Thessalia venditPhiltra, quibus valeant mentem vexare mariti.

FILTH ATION, in chemistry, as well as in domestic (economy,is the process of straining or filtring liquors by means of woollencloth, cotton, linen, paper, or other materials. It deserves to bepreviously remarked, that in every attempt at purifying fluids intiie manner here alluded to, we can divest them only of those fo-reign ingredients which are mixed with them, and not of such asthey hold in solution. The former may be separated from them,by proper filtration ; but the kilter must be disengaged, either byprecipitation or distillation. Although the utility of filtration isthus limited to the noxious particles mixed with liquid bodies,such as foul water, yet it is sufficiently important to deserve someattention. The common filters are of two Sorts ; namely, simplepieces of paper, or cloth, through which the fluid is passed ; orsimilar materials twisted up in the same manner as skeins or wicks ;they are lirst wetted, then squeezed, and one end put into the ves-sel, which contains the liquor to be filtrated ; the other end is tobe suspended beneath the surface of the liquor, the purest parts otwhich drop gradually out of the vessel, leaving behind the coarserparticles. These filters, however, are not calculated for domesticuse : lienee ditierent machines have been invented for the purposeof purifying turbid water. But among these various contrivances,few appear to possess the advantage of simplicity, combined withthat of affording an ample supply of a fluid so essentially neces-sary to the preservation of health. A patent has lately beengranted to Mr. James Peacock, of Finsbury-squate, for a filteringmachine, which is stated to be superior to any hitherto invented.It completely accomplishes the purpose of filtration, by causingthe turbid fluid to ascend through a medium of fine gravel, byprogressive degrees of fineness, by which means the.foulest wateror other fluid becomes perfectly freed from all (mixed) impurities,without any noxious mineral quality, which pumice or other com-mon filtering stones are suspected to communicate. Should, fromcontinual use, its operation become in any degree impeded, itmay be completely cleansed with the greatest facility in the shortspace of one minute: an advantage possessed by none of the com-mon machines that operate by descent. Beside these useful pro-perties, Mr. Peacock’s filtering machine does not occupy moreroom than a large drip-stone with its apparatus, and yields a con-stant and pure stream of more than 300 gallons in 24 hours. Aspecimen of this machine is deposited for inspection at Guildhall,London . We shall here recommend a very simple and effectualmachine for purifying water. It is on the plan of the celebratedfiltering machine, erected at Paris , by means of which the foulwater of the river Seine is so completely purified, as (o be divest-ed of ail its laxative properties; its construction is also simple andeasy. When we reflect on the method which nature pursues inthe"filtration of water, we find that such waters as descend fromhills, though passing through sand and rocks, are seldom perfectlypure ; but that those are the most limpid, which by ascending,ooze out on the side of a mountain. . The cause of this differ-ence appears to be owing to the circumstance, that if the wateronly descends through sand, the finest and most weighty foreignparticles gradually penetrate through the sandy slrata ; on thecontrary, when it is forced to rise through sand, all such ponder-ous ingredients settle at the bottom ; because, from their greaterspecilic gravity, they cannot ascend to the top. The lighter par-ticles of fluids, consequently, in both cases remain in the upperstrata of the earth or sand. From these con alterations, ProfessorParrot, jun. of Paris , was induced to give his filtering machinethe form represented in fig. 1, Plate LXX. The principal partof the machinery consists of a square vessel, bent in the form ofan inverted syphon. The curve may he circular, elliptic, or inany other direction. This vessel is filled with line, pure sand, tilltiearly the height of (he dotted line x, y, which denotes the ascentof the water to D, whence it flows into the receiver. The partmarked A, B, should always project above this line, according tothe size of the filtering machine. To A, B, there is attached awoollen bag, which is open at the top, and the lower part of whichouches the sand. It serves the purpose of collecting the coarsestimpurities, and thus preserves the sand for a longer time from be-coming foul. '|' b e bag- therefore, may occasionally he removed,and rinsed in clean water. It is evident, that the water flow's atA through the bag Into the tiltre, and rises at the place marked

D, which is considerably lower than the former. It affords a veryagreeable sight to observe the most limpid fluid penetrating theuppermost stratum of sand, perfectly similar to that oozing troutthe purest natural spring. Professor Parrot remarks that he pro-cured a filtering machine made of block-tin, for ascertaining byexperiments tiie purity and quality ot water, that may thus be ob-tained in a given time. It consisted of the following dimensions;the small diameter B, Is, was eight Paris inches; the large ot thewhole machine, eleven inches - , consequently the thickness ot thevessel A, B, was one inch and a hail - , the breadth of it, twoinches and seven-eighths. 'Idle perpendicular height ot the lowerside, from C, its basis, to llie rim I), whence die water issues, wasfour inches and one-twelfth—the opposite height of the mouth A,B, eight inches and thiee-fourths; and the height of the sand onthe side marked D, was three indies and one-sixth. Although, inexperiments of this nature, much depends on the relative size andpurity of the sand, which necessarily afford ditierent results, yetProfessor Parrot has, after repeated trials, deduced the followingconclusions, which appear to be well-founded. 1. That the dif-ference of the niveau, or water-level, has an essential influence onthe quantity of the purified water thus obtained ; 2. That a pro-longation of the stratum of sand does not considerably diminishthe product of the tiltre, but remarkably contributes to the purityof the fluid. 3. That if the water be forced to pass through thesand with increased velocity, it will he less pure than by allowingit a proper time for its passage ; and, 4. That a machine of tl*edimensions above described, will furnish about three quarts of wa-ter in an hour, or eighteen gallons ill twenty-four hours. Thisquantity, however, being too large in proportion to the size of tliemachine, it is adviseable, either to lessen the difference of the wa-ter-fall ; or, which is still better, to prolong the stratum of sand, inorder to reduce the filtration of tiie'water to half the quantityabove stated, and to obtain it in greater purity. Thus, a filteringapparatus eighteen inches long from A to U, two inches thick,and four broad, would afford every hour six pints of very purewater. It, therefore, so small a machine, containing a very mo-derate stratum of sand, and requiring only a difference of two orthree inches in the height of the water, furnishes a clear and purefluid, it follows that an apparatus on a larger scale, provided w itha bed of sand from five to six feet long, and admitting of a differ-ence from twelve to eighteen indies in the fall of the water, mightbe usefully employed in public wells, hydraulic machines, andeven in camps, for the supply of an army. In the construct ion oflarge filtering machines, Professor Parrot justly observes, thatthey should not be extended in the direction A, C, D, to a greaterlength than is absolutely necessary ; as, in this case, they will notrequire any considerable difference in ike fall and rise of the wa-ter: on tiie other hand, their breadth and thickness may be ac-cordingly increased. Thus, the diameter of such a machine wouldstill more resemble that of a syphon, as is represented in tig. 2.This form might also be adopted for smaller machines, especiallysuch as are designed for travellers, two of whom might be amplyprovided with pure water, and in a very short time, by a vessel ofthe following dimensions: from P to Q, eight inches long ; fromP to R, twelve inches high ; and the whole four inches in breadth.If the form last delineated be employed on au extensive scale,there should be a trap door in the lowermost part marked li, soconstructed that it may fit exactly, and admit no passage to thewater: this aperture would serve only for the removal of thesand, when it is rendered foul by long use. In the smaller ma-chines, intended for travelling, such a door is unnecessary, as theymay be easily emptied of their contents through either of the ori-fices P, or Q. Instead of this addition to the latter, the upperroom, (which in fig. 1, is circumscribed witli Ihe letters B, F, K),might serve as a resen oil'of pure water, that coulcfeither he de-canted, or drawn off by means of a cock applied lo the centre ofthe machine, marked F. M e think, however, this latter arrange-ment, which is proposed by M. Parrot , in many respects objec-tionable, and therefore advise the reader to make use, of the moresimplified construction, lienee we shall only add, that everylittering machine ought to be provided with a cloth cover, to pre-vent the dust from ri.-ing witli the water, without impeding its ni-tration. The great advantages of filtering machines in tiie dif-ferent processes of dyeing, halting, brewing, distilling, ami all thedomestic arts, is exceedingly evident, As no particle of reali nutriment