Popular Science Monthly/Volume 16/February 1880/Ancient Methods of Filtration
By H. CARRINGTON BOLTON, Ph. D.,
PROFESSOR OF CHEMISTRY IN THE TRINITY COLLEGE, HARTFORD.
THE separation of a liquid from solids suspended in it, by straining through some material pervious to the one and impenetrable to the other, was a familiar process in the remotest antiquity. Observation of various processes in nature, such as the purification of water by trickling through sandy soil, or perhaps the accidental passage of rain-water through an outstretched cloth, a garment, or a tent-cover, would obviously suggest the simple expedient. History fails to record the period of the invention or the name of the individual who first put it in practice. Etymological considerations show that filters were early made of fulled wool or felt; the Latin filtrum, "a filter," being closely connected with feltrun, "felt," or compressed wool, and both are related to the Greek πιλος, signifying hair.
Several writers on the history of science make casual reference to the operations performed by the early chemists: some state in a general way their acquaintance with the processes of distillation, sublimation, filtration, etc.; others are more explicit. Hermann Kopp, in his exhaustive "Geschichte der Chemie," states that "filtration as a chemical process was first accurately described by Geber, who calls it by a special name, destillatio per filtrum, 'trickling through a filter,' in contrast to the collection of a liquid by ordinary distillation." Ferdinand Hoefer, in his "Histoire de Chimie" writing of Geber, mentions that he recognizes two kinds of distillation—with and without fire—the former being "per alembicum," and the latter consisting of "une simple filtration."
Now, we propose to show that the ancients carried on the operation of filtration in two ways, essentially distinct in principle and in the manner of execution, and that these methods were characterized by two different expressions which have been confounded by the authors named. Moreover, we shall establish this by quotations from writings covering a period of more than two thousand years.
In the first place, an examination of the very passage in Geber's Works, referred to by Kopp and by Hoefer, shows that the method therein described differs radically from filtration as ordinarily conducted at the present time. We quote the passage as found in the works of Geber, "the most famous Arabian Prince and Philosopher," "faithfully englished by R[ichard] R[ussell]," and printed at London in 1678. In the thirteenth chapter of the fourth part of the first book of the "Summe of Perfection," Geber treats of the three kinds of distillation: by an "Alembeck," by a "Decensory," and "by Filter." After describing in quaint language the well-known method of using the alembic and the decensory (which differs chiefly in the application of heat on the top of the apparatus), Geber writes thus of filtration: "The Disposition of that which is made by Filter is, that the Liquor to be Distilled be put into a Stone Concha, and the wider part of the Filter put into the said Liquor, even to the Bottom of the Concha, but the narrower part of it hang out over the Orifice of the said Vessel. And under that end of the Filter must be set another Vessel for receiving the Distillation. Therefore when the Filter begins to Distill, the Water with which it was moistened will first Distill off; which ceasing, the Liquor to be Distilled succeeds. Which Liquor, if it be not as yet serene, it must so often be put into the Concha again and redistilled, as until it be Distilled most serene."
This dates from the eighth century, and evidently describes a sort of capillary siphoning. The expressions placing the wider part of the filter into the liquid and allowing the "narrower part of it to hang out over" the vessel admit of no other interpretation. For convenience of distinguishing this method of filtration from that in which porous sacs are employed, we propose to name the former anethisis, a word made from ává, "upward," and ᾖθισις, "a straining off." A study of the chemical works of the middle ages further shows that the expression "destillatio per filtrum" is invariably used to describe anethisis, while "filtratio" is applied to ordinary filtration. We shall give quotations proving this, but first make brief reference to early records.
The ancient Egyptians portray in the rock-cut memorials the operation of filtration in connection with the manufacture of wine. Their simple wine-press consisted of a bag in which the grapes were placed, and squeezed by means of two poles turned in contrary directions. Small colanders of bronze have been found at Thebes. Views of the interior of an Egyptian kitchen, cut in the tomb of Rameses III. at Thebes, represent siphons in use for drawing off liquids of various kinds. (Wilkinson.) The ancient Romans employed strainers and colanders (colum) made of a great variety of materials. Wine-strainers were made of silver and bronze; the poorer classes used linen, and, where nicety was not required, they used those made of broom or of rushes. Strictly speaking, however, percolation through colanders is not filtration, for capillary action plays no part.
It is interesting to note that the earliest mention of filtration which a brief search has disclosed refers to the method we have ventured to call anethisis. This occurs in Plato's "Symposium," written about four hundred years before the Christian era; the passage is as follows: "Socrates then sitting down, observed, 'It would be well, Agatho, if wisdom were a thing of such a kind as to flow from the party filled with it to the one who is less so, when they touch each other, like water in vessels running by means of a thread of wool from the fuller vessel into the emptier.'"
Aristotle, the pupil of Plato, in his essay "De Generatione Animalia," refers to the other process in the following words: "Flesh is produced, therefore, through the veins and pores, the nutriment being deduced in the same manner as water through earthen vessels not sufficiently baked."
This passage, together with others occurring in Plato, shows that both systems of filtration were employed at that early period.
Geber, whose clear description of anethisis we have quoted, was followed by the celebrated Arabian physician, Rhazes; he uses the same expression, "destillatio per filtrum," in the following passage: "Dissolve as much [common salt] as you wish in five times as much warm soft water, and distill per filtrum and congeal [i. e., crystallize]." Rhazes died about 930 a. d.
Among the early writers on alchemy, no one is oftener quoted than Raymund Lulli, surnamed the Enlightened Doctor (born 1235, died 1315 a. d.). In his works we find the following characteristic passage: "Take, in the Name of God, great Bay Salt, as it is made out of the Sea; take a good quantity and stamp very small into a stone-morter: then take Cucurbites of Glass and pour your Salt therein: then take fair Well-water, and let your salt resolve into cleer water; being all dissolved then distil it by Filter; that is to say, hang a jag Felt or Woolen cloath in the Cucurbite; and let the other end hang in another Glass beside it, set as it were under it, that the water may drop into it that the Felt or Cloath may draw out and that shall be cleer as silver."
This unmistakable description of anethisis occurs in the first chapter of a booklet bearing the following title: "Philosophical and Chymical Experiments of the Famous Philosopher Raymund Lulli. . . . wherein is contained. . . . the admirable and perfect way of making the great Stone of the Philosophers as. . . . sometimes practised in England by Raymund Lulli in the time of King Edward the Third." London, 1657.
Thomas Aquinas, the eminent scholastic teacher of the thirteenth century, who is best known by his theological and metaphysical works, also paid some attention to scientific pursuits, possibly acquiring this taste from his learned master, Albertus Magnus. Aquinas, or the divine Thomas, as he was called by his admirers, defines distillation to be the "purification of waters falling drop by drop, and effected by placing a filter cut in the shape of an iron dart in the little dish containing the water to be distilled."
Libavius, in his remarkable work, "Alchymia," sometimes called the first text-book of chemistry (published in 1595), devotes two entire chapters to the subjects of distillation and of filtration. In the fourteenth chapter he describes, with much attention to detail, the manner of filtering by means of pieces of felt (lacinia) shaped like an ox-tongue, the broader portion of which is placed in the vessel containing the liquid to be filtered, and "the apex in the recipient, or, if the vessel has a narrower neck, in a suitable funnel." This demonstrates that the method was not resorted to on account of the want of proper funnels, and suggests that perhaps a special virtue was attributed to a liquid thus purified.
Libavius's work contains rude woodcuts illustrating different methods of procedure. For perfecting the purification a series of four vessels was used. These were placed on steps, one above another, and the liquid passed through a capillary siphon from the uppermost to the one immediately below, and thence by another siphon to the third vessel, and so on to the fourth. This series of vessels can be inclosed in a glass-covered box for filtering volatile liquids. Another woodcut represents the lower end of a capillary siphon hanging into a funnel inserted in the neck of a flask. The method thus clearly portrayed is called destillatio per lacinias, and is evidently regarded as a process of distillation; ordinary filtration through porous stones and through bibulous paper is treated in another and following chapter.
Libavius makes reference to "Hippocrates's Sleeves," by which name were designated conical felt bags used in filtration.
Sir Robert Boyle, in his "Experiments touching the Spring of the Air," writes as follows: "Some learned mathematicians have of late ingeniously endeavored to reduce filters to siphons, but still the true cause of the ascension of water and other liquors, both in siphons and in filtration, [requires] a clearer discovery and explication." And in another place he gives this "explication": "The parts of the filter that touch the water being swelled by the ingress of it to their pores are thereby made to lift up the water till it touch the superior parts of the filter that are almost contiguous to them; by which means, these being also wetted and swelled, raise the water to the other neighboring parts of the filter till it have reached the top of it, whence its own gravity will make it descend."
These passages can only apply to anethisis, which was apparently a common method of filtration in Boyle's day.
Again, to trace this process still later, Juncker, in his "Conspectus Chemiæ," published in 1730, describes seven kinds of filtration. These differ chiefly in respect to the materials used: two methods, however, are essentially distinct; the one is styled "filtratio per chartam bibulam. . . in fundibulo vitreo" (filtration through bibulous paper in a glass funnel), and the other is described in the words "per segmentum panni lanei vel laciniam bombycinam vel funiculos gossypii" (through shreds of woolen cloth, silken threads, or through little strings of cotton).
Our friend Professor S. A. Lattimore sends us another reference to this process from "The Laboratory or School of Arts, etc., compiled by G. Smith, sixth edition, London, 1799" (vol. i., p. 435); the passage is as follows:
"To separate Water from Wine.—Put into the cask a wick of cotton, which should soak in the wine by one end and come out of the cask at the bung-hole by the other; and every drop of water which may happen to be mixed with the wine will still out by that wick or filter."
Thus we see that, so recently as the close of the last century, anethisis was accounted a practical method of filtration.
We have not found this method in Boerhaave (1727), nor in Lemery (1675), nor does Faraday, in his "Chemical Manipulations," make any allusion to it. As a process of purification of solutions, it seems to have been lost sight of in modern laboratory practice.
We have made trial of the method rendered noteworthy by more than two thousand years' practice, and find that, while the process is quite slow, it has certain advantages. Chief among these is the fact that, when the capillary siphon is adjusted, it requires no further attention; there is no pouring into a constantly emptying funnel, and there is no possibility of overflow. When properly arranged, the last drop of liquid passes from the upper vessel to the lower, and, except in certain cases of extreme divisibility, the filtration is perfect. The process seems particularly adapted to the purification of weak solutions, as of mineral waters, where the insoluble portion is not to be conserved. There is obviously much choice in the material of which the capillary siphon is made. We have tried cotton, wool, lamp-wick, and asbestus, and we find stout, silky wads of the latter most serviceable; it is also useful for filtering very acid and alkaline solutions.
The rapidity of filtration does not seem to be hastened by lengthening the longer arm of the siphon, and is chiefly dependent on the number of threads in the filter, and on their fineness. In one experiment, thirty-two strands of cotton yarn filtered twice as fast as sixteen strands, and only half as fast as sixty-four strands. Oil filters much slower than saline solutions, and the latter much slower than pure water.
We have used the expression "capillary siphoning" in describing anethisis, and perhaps it needs justification. The ascension of the liquid is due to capillarity, and the descent through the longer arm of the siphon is in obedience to gravity. Siphoning is dependent on atmospheric pressure, and can not be strictly applied to the present case; yet we find by experiment that, if the lower arm of the woolen threads be raised to the level of the liquid to be filtered, the action ceases, and the form of a siphon is at all events essential to the process.
The quotations from ancient authors show that they must have been quite familiar with capillary attraction, yet the first observance of this phenomenon is attributed by some authorities to Franciscus Aggiunti, physician to the Grand Duke of Tuscany, about the beginning of the seventeenth century, (Aggiunti died 1635.)
In conclusion, the object of this paper is not to propose a return to this ancient method of filtration, the modern rapid processes being more in accordance with present needs; but we have thought it not altogether useless to call attention to a neglected process which can in certain cases be employed with advantage. Where the object of filtration is to collect the insoluble portion, it is obviously of no value; whether the process could be advantageously used on a large scale remains to be tested.