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*0156

9.7

5 ̊5

9'0

Lambeth

Kent

Other Companies.

New River

East London

South Essex

38.08 1'04 177 27.4 27.59 0'73 *0421 156 21'I 29'14 2:42 1341 132 22.2 40'54 1'44 0140 8.2 18.3 2615 The table may be read thus-100,000 lbs. of the Chelsea water contained 24'98 lbs. of solid matter, of which 183 lbs. of organic and other matters were driven off by incineration. 1606 lbs. of oxygen were required to destroy organic matter in the said quantity of Chelsea water. Of the solid matter 145 lbs. are carbonate of lime or its equivalent; of which 7.8 lbs. are got rid of by boiling, and 6'7 lbs remain.

Dr. Whitmore's Report on the quality of the water supplied in St. Marylebone in November, 1865:

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As might have been expected from the late heavy rains, the quantity of organic matter contained in the Thames companies' water during the past month has been greater than in the month preceding; but, considering that this impurity must necessarily, for the most part, be vegetable, the deterioration of the water is of less importance than if it were produced by animal matter. The boasted superiority of the water from Loch Katrine for drinking purposes, which is found to contain no organic matter, and not more than two or three grains of inorganic, and which now supplies the city of Glasgow, has to be proved by the test of time. There can be no doubt that any amount of organic matter in the water we drink, however small, especially animal, is objectionable; but as regards the mineral constituents found in the rivers and wells in and near London, of which salts of lime constitute a large proportion, it is a subject well worthy of consideration to determine whether such water as this does not contribute to supply the earthy matter which gives solidity and hardness to the bony structure of the human frame; for, if not, it seems difficult to tell from what else in our daily

sustenance it is derived.

Essence of Cognac and of Wine.-This is a mixture of several ethers of the ethylic series, but of which the special odour is that of pelargonic ether. The essences may be prepared in two ways: the first gives nearly pure

The degree of hardness hitherto employed by chemists is that first proposed by Dr. T. Clark-viz., one grain of carbonate of lime, or its equivalent, in one imperial gallon of water, or one part in 70,000. The degrees of hardness used in the above table are readily converted into Clark's degrees by multiplying by 7, and then moving the decimal point one place to the left.

pelargonic ether; the other, mixtures of very variable composition, and apparently inferior in quality. By the first method, pelargonic acid is obtained by treating oil of rue by nitric acid; to etherise pelargonic acid, dissolve it in concentrated alcohol, and pass into the mixture a current of dry hydrochloric acid; the pelargonic ether rises to the surface as it forms. By the second method, a fatty body is treated by nitric acid, and fixed fatty acids are produced, such as adipic, pimelic, lauric, succinic, &c., and also volatile acids, which may be distilled, and of which the chief are butyric, valerianic, capric, caproïc, caprylic, œnanthylic, and pelargonic. This is the mixture which is to be etherised. Alcohol is sometimes scented with the product obtained by etherising cocinic acid, extracted from cocoa-nut oil; to obtain this acid, saponify cocoa-nut oil by potash, decompose the soap by hydrochloric acid, dissolve the acid thus obtained in alcohol, and pass into it a current of dry hydrochloric acid; a yellowish liquid will be the result; wash it with water and with alkaline water, when pure cocinic ether will remain, which mix with ten times its volume of alcohol. The richness of commercial essences in pure essences may be ascertained by distillation; alcohol boils between 80° and 85°, and the essences remain as residue. Artificial essences are not generally used in perfumery, excepting essence of mir bane; but other agreeably-scented essences will very probably be some day used, carefully combined and considerably diluted. As found in commerce, they have an odour which is far from agreeable, and they, moreover, have an injurious effect on the animal economy when inhaled in sufficient quantity; they must then, if used, be used sparingly.-Des Odeurs, des Parfums, et des Cosmetiques.

Illuminating Gas from Apples.-A new use for the marc from the cider presses has been discovered by MM. Gouverneur, Butler, and Eichebrenner, who submit it to dry distillation, and so obtain acetic acid, tar, and a large amount of gas of fair illuminating power.—Resumé Oral, &c., par M. L'Abbé Moiyno.

Use of Ultramarine in Refined Sugar.-M. Monier writes to Les Mondes that the use of indigo, referred to in the CHEMICAL NEWS some time ago, has long been given up, and ultramarine is now employed. For a boiling of 800 loaves, weighing on the average 10 kilogrammes each, about 40 grainmes of ultramarine is sufficient. This quantity, which gives about 6 centigrammes to a loaf, is enough to communicate the very slight blue tint required. Ultramarine is perfectly innoxious.

ANSWERS TO CORRESPONDENTS.

All Editorial Communications are to be addressed to the EDITOR, and Advertisements and Business Communications to the PUBLISHER, at the Office, 1, Wine Office Court, Fleet Street, London, EC. Private letters for the Editor must be so marked.

In publishing letters from our Correspondents we do not thereby adopt the views of the writers. Our intention to give both sides of a question will frequently oblige us to publish opinions with which we do not agree.

Vol. XI. of the CHEMICAL NEWS, containing a copicus Index, is now gold-lettered. The cases for binding may be obtained at our Office, ready, price 118. cd., by post, 11s. 6d., handsomely bound in cloth, price 1s. 6d. Subscribers may have their copies bound for 2s. 6d. if and II. are out of print. All the others are kept in stock. Vol. XII. sent to our Office, or, if accompanied by a cloth case, for 18. Vols. I. commenced on July 7, 1865, and will be complete in 26 numbers.

Derby.-Bunsen and Kirchoff, translated by Dr. Roscoe, but the latest discoveries are not contained.

H. M.-A constant stream of ozonised air can be obtained by means of the apparatus described at page 11, vol. x., of the CHEMICAL NEWS. Clericus-We do not remember to have met with any account of the experiments.

Copying Ink-Sugar candy, an ounce; rich black ink, one and a-half pints; red ink, cochineal, oxalic acid, and gum arabic.

phosphorus paste; or of nitre, oxysu phuret of antimony, and flour, Vesuvians are made of nitre and charcoal tipped with the usual tipped as before.

NEWS

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On the Manufacture of Commercial Carbonate of Ammonia, by J. CARTER BELL, F. C.S., Associate of the Royal School of Mines.

THIS salt must have been known even to the alchemists, as it forms one of the chief constituents of putrid urine, but there seems to be no evidence that it was manufactured previously to this century; indeed, one would hardly think that they knew the difference between ammonia and its carbonate. The real difference was first pointed out by Dr. Black, of Edinburgh. Carbonate of ammonia is formed by the putrefaction of animal substances, and by the destructive distillation of animal matter.

In the destructive distillation of bones, carbonate of ammonia is produced, and also water with the oil called "Dippel's oil," with some incondensible gases. The condensed liquors from the carbonisation of the bones are separated into two distinct states-the oily and the aqueous products-the latter containing the carbonate of ammonia; the salt can be separated by sublimation. Many processes have been tried for the manufacture of this compound. Iam informed by Mr. John Hogarth (who has been engaged in this chemical operation for forty years) that in the year 1825 a Mr. Holmes manufactured this sult in the old Haymarket, Liverpool; it was made from stale urine, and the resulting blocks were very small, weighing about six pounds. At the present time the weight of a block is about two hundredweight. On March 11, 1844, Dr. Wilton Turner took out a patent for obtaining salts of ammonia from guano, The guano is subjected to destructive distillation in close vessels at a low red heat during the greater part of the operation, but the temperature is increased towards the end. The products of distillation are collected in a series of Woulfe's bottles, by means of which the gases evolved during the operation may be made to pass two or three times through water before escaping into the air. The products consist of carbonate of ammonia, hydrocyanic acid, and carburetted hydrogen; the first and second are rapidly absorbed by the water, with the formation of a strong solution of hydrocyanate and carbonate of

ammonia.

In 1849 Mr. Hills took out a patent for obtaining carbonate of ammonia from guano. To effect this the guano is first mixed with charcoal or powdered coke; the mixture is then heated, and the carbonate obtained by sublimation. Peat has been experimented on for the production of this salt; whether it will be an economical process remains to be proved. Mr. Hills took out VOL. XII. No. 317.-DECEMBER 29, 1865.

a patent for obtaining ammonia from peat, and Mr. Rees Reece in 1849 also had a patent very much like Mr. Hills'. The first part of the patent is for an invention for causing peat to be burned in a furnace by the aid of a blast, so as to obtain inflammable gases, tarry and other products. The tarry products may be employed to obtain paraffine and oils for lubricating machinery, &c., and the other products may be made available for evolving ammonia, wood spirit, and other matters by any of the existing processes. On July 27, 1849, a statement was made in the House of Commons to the effect that 100 tons of peat would produce 2602 pounds of carbonate of ammonia.

In 1841 Mr. Laming took out a patent for manufacturing carbonate of ammonia by mixing its separate acid and alkaline constituent, instead of by the decomposition of ammoniacal salt. One of the processes used is to cause ammonia and carbonic acid gas obtained separately from any convenient sources to traverse a succession of leaden chambers maintained at as cool a temperature as may be conveniently practicable, and so continued as to favour the admixture of the dissimilar gases. In this process it is not essential that the two gases be present in their combining proportions; it is preferable that the carbonic acid be in greater abundance than will combine with the ammonia which is present. Sometimes a stratum of water, or of water impregnated with ammonia, is placed in one or more of the leaden chambers. Carbonic acid and ammonia in the form of gas are then introduced; in which case, it is stated, a larger proportion of carbonic acid gas is found in the resulting salt, or saline solution, than when only the hygrometric moisture of the aëriform fluid is present. Mr. Laming also converts the hydrosulphate of ammonia contained in gas liquors into carbonate of ammonia by the following process: A mixture of deutoxide of copper and charcoal, or other form of carbon in fine powder, in the proportion of twelve parts by weight of the former to one of the latter, is introduced into a retort made red hot and furnished with an eduction pipe which passes through cold water, and finally enters into the gas liquor. The formation of carbonic acid gas soon takes place by the union of the carbon with the oxygen of the metal, and this gas combining with the base of the hydrosulphate of ammonia converts it into carbonate, with liberation of sulphide of hydrogen. When the carbonic acid ceases to come away, nearly all the carbon will have disappeared from the retort, and the oxide of copper reduced to the metallic state. charge is then drawn, and left to cool while a second charge of similar materials is being worked off; during which time the copper reabsorbs oxygen from the air, and becomes again deutoxide of copper, which may be used anew with fresh carbon.

The

Messrs. Crane and Tullien, in their patent of January 8, 1848, describe a method of manufacturing ammonia in the state of carbonate, hydrocyanate, or free ammonia by passing any of the oxygen compounds of nitrogen, together with any compound of hydrogen and carbon, or any mixture of hydrogen with a compound of carbon, through a tube or pipe containing any catalytic or contact substance. The substance which is preferred is platinum in the state of sponge, or asbestos coated with platinum. This is to be placed in a tube and heated to about 600° F., so as to reduce the temperature of the product, and at the same time prevent the deposition of carbonate of ammonia, which passes onwards to a vessel of the description well known and employed for the purpose of condensing carbonate of ammonia. The

NEWS

Dec 20, 1865

condenser for this purpose must be furnished with a safety pipe, to allow the escape of uncondensed matter, and made to dip into a solution of any substance capable of combining with hydrocyanic acid or ammonia. Carbonate of ammonia is manufactured at the present time from a mixture of sulphate or chloride of ammonium and common chalk, heated in retorts and sublimed. The decomposition of the chloride of ammonium may be represented thus:

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In the manufacture of ammonia alum, the ammonia is derived from gas liquor; the liquor is heated, and nearly all the volatile ammonia driven off. The residue is taken out of the boilers and used for the manufacture of carbonate of ammonia. It is treated with a little acid till it is neutral, then evaporated by means of heat in large hemispherical iron pans set in brickwork. When it has arrived at the crystallising point, it is allowed to cool, and crystals are then deposited; or the hot liquor may be run into other coolers for crystallisation. The mother liquor is syphoned off, and then the inside of the pan is seen studded with in tense black crystals, of the prismatic form when sulphuric acid has been used for the neutralisation, and cubical with hydrochloric acid. The crystals are now shovelled out of the pan, and washed with the mother liquor. They are then re-dissolved, the liquor run into coolers, and re-crystallised. In the re-dissolving a great deal of sediment is deposited, consisting chiefly of the matter mechanically locked up in the crystals. The crystals when dry are of a dirty white colour; they are now ready for the next operation of converting them into carbonate of ammonia. For this purpose cast-iron retorts, the shape of an elongated muffle, are used. The neck of the retort is round, and closed with an iron door, kept in its place by means of a screw. The retorts are about seven feet long and one and a-half deep. Three are set in brickwork in the form of a triangle, and heated by one fire. They communicate by means of iron pipes with a leaden chamber which is technically called a balloon. It is about six feet high, eight long, and two and a-half wide. These balloons are supported upon scaffolding so as to be on a line with the retorts, and are kept in their places by means of iron bands. At the bottom of each balloon is a small pipe, which is always kept open to allow for the escape of steam, and water highly charged with carbonate of ammonia. There is a constant dropping from this pipe, which is collected in a pail, and re-sublimed. If this pipe were not there, the pressure inside the balloon might cause it to be blown off the scaffolding. Great attention has to be paid to the heating of the retorts. If they were heated too strongly, most disastrous results might occur.

The retorts are charged once every twenty-four hours with a mixture of carbonate of lime and ammoniacal salt; the chalk is well dried on an iron plate which is set over the flue, so that the waste heat of the fires economically dessicates it. All the retorts are not charged at the same time, for often there are five and six sets; if they were, the labour would be too great, and a greater number of men would be required; but to do away with that difficulty one retort in each set is charged at the same hour every day; the first charging takes place at seven, the second at eleven, and the third at three, and by that time the whole of the retorts have been charged. The contents are frequently stirred up with long iron rods (which are pushed through holes made in the door of the retorts) to assist the desomposition. Before a new charge is put in, the pipes

leading to the balloons are well cleaned out, as they are very liable to become stopped up. The used-up charge, which consists principally of chloride of calcium, is drawn out into an iron barrow and wheeled away to some waste ground; the new charge (which is generally two of chalk to one of the salt) and which has been carefully weighed and well mixed, is thrown quickly into the retort, the door is luted on, and then the retort is left for twenty-four hours, the contents receiving an occasional stir.

When the retorts have been worked for about fourteen days, the balloons are opened, and the impure carbonate is found as a thick crust lining all the sides; it is deposited in different coloured layers, according to the impurity of the carbonate. The chief impurities will be carbonate of lime and chloride of calcium, which are carried over mechanically; the salt is well scraped down from the sides, and the balloon prepared for another fourteen days' operation. These balloons have to be of considerable size, or there would be much waste from the salt being carried off by the steam; in each balloon is a small test-hole, closed with a plug of wood, this is for telling how the sublimation is going on. The impure carbonate is all collected and taken to the resublimation pans. The salt is put into iron tanks about sixteen feet long and two and a-half deep; they are wider at the bottom than at the top, being two feet seven inches at bottom and two feet at top. These tanks are closed with two plates of iron with four holes in each, about one foot in diameter and one foot apart from each other. Over every hole is placed a conical leaden vessel with a flat top. These vessels are formed of a sheet of lead, and the two ends are kept together by means of staples and wedges; a circular piece of lead is luted on the top of these receivers; the height of them is about two feet. The tanks are set in brickwork, with a fireplace at each end. They are charged every fortnight; a certain quantity of water is first put in, then the impure carbonate. The receivers are all luted on over their respective holes, and a small fire made at each end of the tank. Great care is required in regulating the temperature, because the heat must not be too high, as the salt sublimes from 120° to 130° Fahrenheit. In the end receiver is a small hole closed by a plug; on taking this out it can be seen whether the temperature is too high; if it is, the fires have to be damped. A thermometer is generally used, but some people prefer to trust to their own judgment. Instead of the tank and fires separate pots may be used, each one being surmounted by a leaden cap; these pots are either set in brickwork and heated by the flue of the retort furnace, or they may be set in a water bath. At the end of fourteen days the leaden receivers are lined with a thick crust of car

bonate; they are taken down, and the lead stripped off; the outside of the block is rather dirty, it is well scraped, and then broken into pieces, packed in jars, and sent to the market. The leaden receivers are well washed and reshaped. A small quantity of the residuary liquor is taken out of the tanks, but the chief part is left in, a fresh charge of carbonate is added, the receivers are luted on, and the operation goes on the same as before.

The greatest use which is made of this salt is by bakers and confectioners; it is largely employed in medicine and in the manufacture of smelling salts.

Royal Institution of Great Britain.-The fol

lowing are the lectures for the ensuing week:-Tuesday, Jan. 2, Thursday, Jan. 4, and Saturday, Jan. 6, 3 o'clock, Professor Tyndall, "On Sound" (juvenile lectures).

NEWS

PROCEEDINGS OF SOCIETIES.

CHEMICAL SOCIETY.

Thursday, December 21.

Professor W. A. MILLER, M.D., F.R.S., President,

in the Chair.

THE minutes of the previous meeting were read and the several donations to the Society's library were acknowledged in the usual manner. Mr. William J. Barnes was formally admitted a Fellow, and the following gentlemen were duly elected by ballot-viz., John Percy, M.D., F.R.S., lecturer on metallurgy, Royal School of Mines; Mr. Ernest T. Chapman, 25. Somerset Street, London; Mr. Charles N. Ellis, Bow Common; and Mr. Thomas Ward, Mechanics' Institution, Bolton. The names of the candidates read for the first time were,- Mr. Edward Purser, jun., 116, Fenchurch Street; Mr. William Thorpe, 13, York Terrace, Kingsland Road; Mr. Arthur E. Davies, Surgeons' Hall, Edinburgh; and Mr. Franklin Epps, Great Russell Street; and for the second time the names of the following,-Mr. Thomas B. Redwood, 19, Montague Street, Russell Square; Mr. John Conroy, Christ Church, Oxford; Mr. James Speir, Newcastle-on-Tyne; and Mr. Robert Henry Smith, Rodney Street, Pentonville.

The PRESIDENT said he was glad to notice among the visitors the presence of several gentlemen who had given attention to the subject of Mr. Yates' paper; letters of invitation had likewise been addressed to the Astronomer Royal and other eminent authorities, who were, however, unable to attend.

A communication entitled, "On the Best Material for Mural Standards of Length," was read by Mr. JAMES YATES, M.A., F.R.S. The author referred to a previous essay on this subject which he presented to the British Association for the Advancement of Science, and had the honour of reading at the Birmingham meeting in September last. His communication was followed by others stating the advantages of the metric system, and gave rise to an animated discussion, in which Dr. Williamson and other eminent chemists took part; and the result was that the Association appointed a committee for the purpose of promoting the extensive use of the metric system in scientific documents, the teaching of the system in schools and colleges, and for the general information of the people. The author's previous remarks on mural standards were arranged under the following heads-The material; the form and dimensions; the description by means of letters, figures, and other marks; the distribution and exposure to public view; the use in education; the aids to be afforded by the British Association. To the first of these heads-viz., the material-the author proposed on the present occasion to limit himself, and he wished now to have the opinion of the Fellows of the Chemical Society upon this point. He had already offered (at Birmingham) a suggestion that Baily's metal be employed, but since that time doubts had been expressed in regard to the permanence of this metal under the usual conditions of atmospheric exposure. Baily's alloy consisted of copper sixteen parts, tin two and a-half, and zinc one part, and the use of it had been recommended in the "Act for Legalising and Preserving the Restored Standards of Weights and Measures," 18th and 19th Victoria, cap. 72, and actually adopted by the Royal Commissioners in 1843 under the supposition that it could not rust or form a loosely adherent oxide. An adverse opinion having lately been expressed, the author proceeded to criticise the respective advantages of a variety of other metals, alloys, and compound bars, which seemed to be capable of employment for these purposes, and vocated-if no scientific reasons should appear to the contrary-the use of ordinary brass covered with a thin coating of gold. The metals should be drawn out together,

for the twofold purpose of hardening the gold and securing its perfect adhesion to the brass, and fine lines or divisions could then be engraved upon the face of the bar in such a manner as to remove the film of gold in those parts. By the action of the air upon the exposed portions of brass, distinctly marked lines would soon be formed of a black or greenish-bronze colour upon the bright gold ground. A bar, very attractive and ornamental in its character, would thus be produced, and one which the author considered to be admirably adapted for use as a mural standard, and by placing the yard and metre in apposition, would be likely to find favour and inform the minds of the British public. Several of these could be prepared and set up on the outside walls of public buildings both in the metropolis and in the provinces, if the cost of from three to four pounds each would not be judged excessive. Mr. Yates then referred to the possibility of reducing the cost by the substitution of a coating of platinum, or of an alloy of platinum and iridium, for the gold; and proceeded to enumerate other suggestions relative to the employment of the best kind of aluminium bronze (60 parts copper and of aluminium); speculum metal, improved by the addition of a little arsenic; and lastly, for in-door applications, of steel, which had the sanction of the Rev. R. Sheepshanks, although it was well known to be liable to rust. The author stated that he noticed rust upon the steel standard last referred to, even whilst it was on view at the Paris Exhibition of 1855, and he believed its condition could now be ascertained by a visit to the Conservatoire des Arts et des Metiers. The author concluded by referring to mural standards already open to public inspection on the outside walls of the Royal Observatory, at Greenwich, and of Messrs. De la Rue's manufactory, in Bunhill Row; by exhibiting some accurate metre scales prepared by Mr. James Gargory, 41, Bull Street, Birmingham, and by Messrs. Elliott Brothers, of the Strand, London, besides lithographic representations of the same; and invited discussion upon the several points raised in his paper, promising for himself and colleagues to submit a statement embodying the chemists' decision to the ruling authorities of the British Association.

40

The long and interesting discussion which followed the reading of Mr. Yates's paper will be reported next week. Before adjourning the meeting, the PRESIDENT moved a vote of thanks to the author, and announced that on the next occasion, January 18th, Dr. J. H. Gladstone would read a paper, "On Pyrophosphotriamic Acid," and that on 1st February Dr. Gilbert would address the Society upon an agricultural subject.

PHARMACEUTICAL MEETING.
Wednesday Evening, December 6.
(Continued from page 299.)

DR. ATTFIELD read a paper entitled "Observations and Experiments on the Physics of Filtration." Dr. Attfield's paper was of great length, and our space allows us to give but a very short abstract. He began by thus defining the nature of filtration :-" The nature of the operation of filtration, as usually conducted, is so simple that but little has been or need be published concerning it. The variety of circumstances under which filtration is conducted, as well in social as in commercial life, has given rise to many contrivances for effecting the operation; filtering materials are numerous, the forms of the vessels designed to hold the materials scarcely less numerous, and the arrangements to facilitate and perpetuate filtration many and ingenious; but the nature of the operation, as distinguished from the operation itself, is the same, or nearly so, under all ordiad-nary circumstances. Indeed, its nature is identical with that of some operations which, conventionally, are quite distinct from filtration, and which are always spoken of by other names. In the process termed sifting we have the

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same action occurring as in filtration; it might, in fact, be called dry filtration. In the netting of fish we also have the same action. Again, the operation of 'straining' even still more closely resembles that of filtration. The nature, then, of the operations conducted with filters, strainers, sieves, or nets is identical; the operations themselves quite distinct, and, very properly, called by different names. What I wish to speak of now is not the operations themselves, but their nature—that is, the laws which regulate their action, especially in respect to filters." After noticing the resistance offered by various filtering media, he dwelt at some length on the different aids to filtration -viz., (a) hand pressure, (b) lever or screw pressure, (c) hydraulic pressure, (d) atmospheric pressure, (e) hydrodynamic force, or to a combination of these forms of pressure. Under the fifth head, filtration aided by hydrodynamic force, the author described the filter invented by Mr. Schacht, in which the pressure is produced by the influence of a column of water below the filtering medium. With this instrument Dr. Attfield has made numerous experiments, the results of which are of no immediate practical interest. We append the author's summary, which will give our readers an idea of the scope and results of Dr. Attfield's experiments:-" The practical applications of the truths we have been considering are for the most part obvious, and already | well known to all. But of what new value are they? as follows:-Firstly, these observations and experiments give us, I think, clearer, more correct views of the nature of the operation of filtration than most of us had before. We should, I think, regard filtration under any and all circumstances from a hydrodynamic point of view. We should regard it as the flow of a liquid from an orifice in the vessel containing the liquid, the flow being interfered with or resisted to a greater or less degree by a porous fabric, termed a filtering medium. The rate of flow we should regard as normally following that described in the theorem of Torricelli-namely, in proportion to the square root of the distance from the orifice of outflow to the surface of the filtering liquid; or, as the law may perhaps be stated for our purpose, 'the rate of flow is proportionate to the square root of the power,' whether that power be derived from gravitation, muscular or mechanical force, or the elasticity of compressed air or steam. As the flow becomes slower and slower, the manifestation of this dynamic law becomes less and less evident, and the existence of a static law in the instrument more and more evident, until, when the flow ceases altogether, a static pressure only exists within the apparatus; hydrostatic in the common conical and other simple filters, the filter bag pressed in the various ways, and the filter in which there is a column of liquid above the medium; aerostatic where the air is removed from below a medium or additional air, &c., forced on the filtering mixture from above, or where there is a column of liquid maintained below the medium. As a filtering medium always presents some resistance, dynamic laws can never apparently exclusively obtain in a filtering apparatus, though they nearly do so in the filtration of water for drinking purposes. So also, as that resistance can never be complete, filtration can never be a static operation, nor can static law exclusively obtain in a filtering apparatus, until the latter ceases to be a filter, though they nearly do so when the filtered liquid is escaping drop by drop, as may generally be seen in an analyst's filter. Though, however, a filtering apparatus can never be the exclusive seat of either dynamic or of static laws, it is quite possible that the flow from the apparatus is governed purely by dynamic laws. The rate of flow does not appear to be a pure dynamic rate. probably because we can only compare it with the total amount of force applied. But a portion of that force is expanded in producing static pressure within the instrument; the residue, if we could estimate it, would probably show that the flow from the filter is actually, #Similarly, " nitration" might be called "wet sifting."

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though not apparently, a flow proportionate to the square root of the amount of force which produces it. For instance, a filter is giving a certain number of drops per minute, under a certain amount of force; double the amount of force, and we get nearly double the number of drops: in the first case, nearly all the force is expended in producing static pressure within the instrument, the residue being expended in producing the flow; in the second case, also, nearly all the force is expended in producing static pressure within the instrnment, but not quite twice as much as in the first case: thus, probably, the residue of power is four times greater than in the first case, and hence we get a double flow. And so on, until, with a free orifice, there is no static pressure at all within the instrument, when we get a rate of flow which is apparently as well as actually dynamic. It is for these considerations chiefly that I think we should regard filtration in a dynamic aspect. A less strong, though more obvious reason, is that useful filtration—that is, rapidity of flowis in proportion to the extent to which dynamic laws obtain in filters. Secondly, we have been told that pressure filters have not hitherto proved of the service in Pharmacy that was expected of them; that where they are most needed—namely, for the separation of solid matter in a very minute state of division or in a flocculent condition, there they fail, and that a turbid instead of a clear and bright filtered liquid results. Now, so long as we consider pressure-filters to be static instruments, this result must be inexplicable. But once realise their dynamic character, and the explanation of the fact would seem to be this, - a flock or particle of solid matter finds itself at the mouth of a pore of a filter; if that particle were the object of static laws only (aërostatic or hydrostatic), there it would remain, resting, so to speak, on the edges of the pore, and there it would remain, we will suppose, if the pore were the pore of a common filter in a common funnel, the pressure that is above the particle being in this case only slightly greater than that below the particle; but now greatly increase the pressure on that particle from above, either directly by adding pressure, or indirectly by taking pressure from below, then the particle is at once shot through the pore, it being compressed if it be a flock, or it itself enlarging the fibrous interior of the pore, if the particle being incompressible, and if the pore be in paper, cotton, wool, &c. In other words, the force which increases the gravitating motion of fluid particles through the pores of a filtering medium, increases the gravitating tendency of any solid particles which may be resting within or on the edges of those pores. This explanation (and the being able to give explanations of facts is a matter of practical value) follows, I think, from the consideration of our subject. In Mr. Schacht's pressure-filter, this stated objection to the old pressure-filters may possibly not obtain, because the pressure can be increased so gradually that the consolidation of the particles of solid matter, which are constantly increasing the resistance of the filtering medium, goes on pari passu with the pressure itself; in other words, the closeness of the filtering medium in his instrument increases regularly with the pressure instead of spasmodically, as in other older instruments. Whether this be so or not, can only be determined by experience in the use of his filter. Thirdly, apply hand pressure, lever, or screw pressure, and hydraulic pressure, directly to as small a portion of the surface of a filter-bag as possible. Fourthly, if, in filtration, pharmaceutists, cngineers, and others desire to have the full benefit which the use of a long column of liquid below their filter gives them, that column must be perfectly continuous, there must be no break in it caused by the introduction of air from without the instrument, or by the accumulation of air coming out of solution in the water, as we all know it will do when atmospheric pressure is removed. The practical means of getting rid of such accumulations Į have already described

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