[H]Degrees of Twaddell's hydrometer may be reduced to specific gravity by multiplying by ·005 and adding 1·, thus 10° Tw. = 1·050 sp. gr.
[H]Degrees of Twaddell's hydrometer may be reduced to specific gravity by multiplying by ·005 and adding 1·, thus 10° Tw. = 1·050 sp. gr.
The impurities of sulphuric acid most common and injurious for tanning purposes are iron and nitrous acid. Iron is detected on neutralising with soda or ammonia, when it falls as a yellowish precipitate, which may be recognised by the ordinary tests (p. 100). Nitric and nitrous acids are detected by pouring a strong solution of ferrous sulphate cautiously on to the top of the strong cold acid, when a dark ring is formed at the junction of the two liquids.
Hydrochloric acidmay be tested with soda solution like sulphuric. 1c.c.of normal soda = 0·0365grm.or 3·65per cent. HCl. It may also be calculated from specific gravity.
The presence of iron is indicated by a yellow colour, and may be confirmed by the usual tests as in sulphuric acid.
Oxalic acidshould be pure white and soluble in distilled or rain-water. 6·3grm.may be weighed out, and made up to 200c.c.If 20c.c.of the solution for a test be used, eachc.c.of normal soda solution equals 10 per cent. of pure crystallised acid, C2O4H2+ 2 Aq. The end-reaction with methyl orange is rendered sharper by the addition of a few drops of neutral calcic chloride towards the end of the titration.
Acetic acidmay be similarly determined, eachc.c.of normal alkali being equivalent to 0·06grm.of C2H4O2. Caustic soda, or lime-water and litmus, give sharper results than sodic carbonate and methyl orange. Brown pyroligneous acid is difficult to test from the dark compounds formed with soda, but may be indirectly determined by the quantity of marble, baric carbonate, or magnesia which it will dissolve (comparep. 100), or very possibly by lime-water like tan-liquors with a little tannin as indicator.
EXAMINATION OF LIME AND LIME-LIQUORS.
The quantity of caustic lime in either quicklime or lime-bottoms may be determined by weighing a quantity of the finely powdered material containing not more than 1grm.of caustic lime, and shaking it thoroughly with 1litreof distilled water and filtering. 100c.c.should be taken, and decinormal acid, sulphuric or hydrochloric (or if oxalic, with addition of neutral calcic chloride, or with litmusinstead of methyl orange as indicator). Eachc.c.of decinormal acid corresponds to 0·0028grm.of CaO. If the filter and residue be treated with sufficient normal acid to dissolve the whole of the carbonates, and then titrated back with normal sodic carbonate and methyl orange, the loss (less soda solution required than acid was originally employed) is equal to the carbonate of lime and carbonate and hydrate of magnesia present. 1c.c.of normal acid = 0·05grm.of CaCO3.
Fig. 17.
Fig. 17.
Fig. 18.
Fig. 18.
Lime-water and lime-liquors may be titrated as above, with sulphuric or hydrochloric acid and methyl orange; but in the latter case ammonia (and if soda ash or "Inoffensive" is used, soda and potash also), and the lime salts of weak organic acids will be estimated with it. It is difficult to get a sharp end-reaction in old liquors from the organic acids (caproic, amidocaproic, &c.) present. To determine the ammonia, 50-100c.c.of the liquor may be distilled in a small retort or flask, and the escaping NH3collected in aU-tube or "nitrogen bulb" (Fig. 17), containing 20-50c.c.of normal acid, which is afterwards titrated back with sodic carbonate and methyl orange. Kathreiner employs the arrangement shown inFig. 18. 30c.c.of the liquor to be examined is placed in a shallow vessel on a piece of ground-glass, and 10c.c.of normal acid in a second cup, which is supported over the other by a glass or wire triangle. The whole is covered with a small bell-glass, of which theedges are smeared with, vaseline. At the end of 24 hours, all the ammonia will have been absorbed by the acid, which is titrated back. The lime-liquor sample should be drawn after well plunging the lime, and rapidly filtered into a flask from a funnel covered with a clock-glass.
Determination of Gelatin and Coriin in Lime-liquors.—This cannot be done directly, though considerable quantities of dissolved hide-substance are precipitated on acidification of the liquor with hydrochloric acid and saturation with common salt. If the liquor be neutralised with hydrochloric acid, and evaporated to dryness on the water-bath, nitrogen may be determined in the residue by combustion, and the hide-substance calculated from it (comparep. 108). This method is serviceable in determining the amount of hide dissolved by different solutions, or under different conditions.
The total solids of lime-liquors are estimated by evaporating 20-30c.c.in a porcelain crucible at 212° F. (100° C.). The organic matter is then found by igniting and determining loss (using ammonia nitrate if necessary to complete the combustion of the carbon). The ash is mostly lime carbonate. Soda, potash, and other bases may be determined in it by the usual methods, if required.
ESTIMATION OF SULPHUR AS SULPHIDE IN SODIUM SULPHIDE, &c.
32·6grm.of chemically pure zinc is dissolved in dilute sulphuric or hydrochloric acid. This is readily accomplished in a flask, if a piece of platinum foil, or a few drops of platinic chloride are added to form a galvanic couple with the zinc. After solution, sufficient ammonia is added to redissolve the precipitate at first formed,[I]and the whole is made up to 1litre. Eachc.c.= 0·016grm.sulphur or 0·242grm.of sodic sulphide. This solution is added drop by drop from a burette to the solution of sulphide, and forms a white precipitate of zincic sulphide. The end of the reaction isknown by placing a drop (with a glass rod) side by side on a piece of white filter paper, with a drop of solution of lead acetate. So long as sulphide remains in solution, it will form a black margin of lead sulphide where the drops touch. The drops must not be placed too close, as the solid zinc sulphide isalwaysdarkened if it comes in contact with lead acetate. It must be noted that tank-waste liquors, and many other sulphur solutions, contain polysulphides which are estimated by zinc, but which do not unhair, at any rate in an unaltered state.
[I]If any brown residue remains, the zinc is contaminated with iron.
[I]If any brown residue remains, the zinc is contaminated with iron.
CHEMICAL EXAMINATION OF LEATHER.
Fig. 19.
Fig. 19.
Estimation of Grease.—To determine oil and grease, a weighed quantity (5-10grm.) of the leather in fine shavings or raspings is exhausted with petroleum-ether (gasoline) in a fat-extraction apparatus, of which a convenient form is represented inFig. 19. The leather is placed in the upper vessel, of which the lower opening is loosely plugged with cotton-wool, and the petroleum-ether in the flask, which is gently heated in a water-bath. The petroleum-ether boils and condenses in the inclined condenser through the casing of which a stream of cold water is passed, whence it drops back into the flask through the material to be exhausted. When the exhaustionis complete (when a drop of petroleum-ether from the leather leaves no grease when allowed to evaporate on a clean glass), the upper part of the apparatus is removed, and the ether is distilled off. If the flask has been previously weighed, it is maintained in an air-bath at 212°-248° F. (100°-120° C.) for some hours, allowed to cool, and weighed, when the gain of weight is the grease and oil. Paraffin would also be extracted and reckoned, and probably traces of resin if present. Ordinary ethylic ether cannot be used, since tannins and many of their products are soluble in it. Probably carbon disulphide might be substituted. Care must be taken to avoid explosion, as the vapours of petroleum are very combustible. The residue left in the percolator may be examined for matters soluble in water, by extracting again with hot distilled water, or for resins (and phlobaphenes) by extraction with alcohol.
Estimation of matters soluble in water.—This is important both to detect weighting, and to draw conclusions as to the materials used in tanning. Fine raspings or shavings may be exhausted with warm water in a percolator, or roughly a weighed piece (20grm.) of leather, air-dry, may be well kneaded and worked in 100c.c.of warm water in a basin. 50c.c.of this may be evaporated to dryness in a light basin over the water-bath (or under a paper hood on a steam boiler), and the gain of weight will give the amount dissolved from 10grm.This is more accurate and quicker than redrying the leather and weighing loss. The residue will contain tannins and their products, often in considerable quantities, and may be examined by the table of reactions,p. 112, though these are as yet very imperfect. It will also contain glucose, dextrin, and soluble salts, if these have been used to give weight and firmness. The absolute proof of weighting with glucose or dextrin is difficult, since tanning materials naturally contain these and analogous principles. The residue may be powdered and exhausted with cold water, and the tannins and colouring matter removed by shaking with magnesia (p. 108) or lead carbonate.Fehling's solution[J]is then added and the mixture is heated nearly to boiling. A rapidly formed and considerable precipitate of red cuprous oxide indicates weighting with glucose or dextrin. Leather extracts, however, invariably reduce Fehling's solution more or less, and a conclusion can only be drawn after some experience and comparative tests. Gallotannic acid and pyrogallol reduce it when heated, but not cane sugar or gum arabic. If a solution of cane sugar be heated to 68° C. for1/4hour with 10 per cent. of fuming hydrochloric acid, it is "inverted," and then after neutralising the acid with potash or soda, will reduce Fehling's solution when heated.
[J]4grm.cryst. cupric sulphate are dissolved in 20c.c.of water; and 16grm.of neutral potassic tartrate and 13grm.of fused sodic hydrate are dissolved in 60c.c.The two are mixed, made up to 100c.c., and boiled for some minutes. It should always be tested before use by boiling a portion, which should remain perfectly clear.
[J]4grm.cryst. cupric sulphate are dissolved in 20c.c.of water; and 16grm.of neutral potassic tartrate and 13grm.of fused sodic hydrate are dissolved in 60c.c.The two are mixed, made up to 100c.c., and boiled for some minutes. It should always be tested before use by boiling a portion, which should remain perfectly clear.
The soluble mineral salts are detected by igniting the residue left after evaporation of a separate portion in a porcelain crucible.[K]From unweighted leather, the quantity is very small. The ash is exhausted with a fewc.c.of distilled water, which will dissolve most sulphates and chlorides, which may be detected in small portions of the solution by baric chloride and silver nitrate respectively. Baric chloride and lead acetate are precipitated by a drop of sulphuric acid, and the latter is blackened with ammonic or sodic sulphide. Lime is precipitated by addition of ammonic chloride, ammonia, and ammonic oxalate; magnesia by the subsequent addition of sodic phosphate (seep. 109). The carbonates in the insoluble part (mostly derived from salts of organic acids) may be taken up by dilute hydrochloric acid and tested separately, or the acid may be used at first. Any residue undissolved by the acid is probably lead chloride, and will be dissolved by hot water.
[K]A platinum crucible must not be used for fear of its destruction by lead, unless this metal has been proved absent.
[K]A platinum crucible must not be used for fear of its destruction by lead, unless this metal has been proved absent.
Estimation of ash.—The leather in small pieces (either after or before extraction with water) is incinerated in a porcelaincrucible. The ash is extracted with hydrochloric acid. The insoluble portion may contain barium sulphate (barytes), lead sulphate, sand, clay, &c. For further examination, ordinary chemical text-books must be consulted. Any large amount of ash indicates weighting. Müntz found only about 0·5 per cent. of ash from bark-tanned leather.
Determination of hide substance.—It is sometimes of interest to determine the proportion of dry hide-substance in a sample of leather, but there is no known means of doing this directly. If, however, the leather be dried, finely powdered by rasping, and the nitrogen determined by combustion, either with soda-lime (Will and Varrentrapp's method), or with copper oxide (Dumas), the hide-substance may be calculated, since tannin contains no nitrogen. Müntz found unhaired skin dried at 230° F. (110° C.) to contain 51·43 per cent. of nitrogen (compare alsop. 20).
DETERMINATION OF FREE ACIDS IN TAN-LIQUOR.
The lime-water method mentioned onp. 172is, from its simplicity, well suited for daily use in the tannery as a control method for ordinary working; but where it is necessary to make very exact estimations, or to determine the various acids separately, it is not so satisfactory as one recently published by Kohnstein and Simand (Dingl. Polyt. Jour., 1885, cclvi. 38).
The acids usually present in liquor consist of several members of the fatty or acetic group, which distil over with boiling water, of other non-volatile organic acids, and sometimes sulphuric acid, which is added to assist the swelling of the leather.
To determine the acids of the acetic group, Kohnstein and Simand proceed as follows:—100c.c.of the liquor are distilled, in a flask or retort with a good condenser, to about 30c.c., allowed to cool a little, made up again to 100c.c., and again distilled; and this is repeated till about 300c.c.have passed over. The distillate is then made up to 300c.c., well mixed by shaking, and the acid is determined with standard soda. Methyl orange and sodic carbonate is not so suitable for this titration, as caustic soda and litmus, since methyl orange is not very sensitive to vegetable acids. If it be desired to ascertain what quantity of acids of the acetic group exist in combination with lime and other bases in the liquor, small excess of sulphuric acid may be added to the residue in the retort, and the distillation repeated, when the organic salts will be decomposed and the volatile acids come over.
To determine the total free organic acids, Kohnstein and Simand shake about 80c.c.of the liquor with 3-4grm.of freshly ignited magnesia, quite free from carbonate and from lime, and allow to stand for some hours with frequent vigorous shaking, till the liquor, which at first is brown or dirty green, becomes almost colourless and gives no reaction of either acid or tannin. The mixture is then filtered, and the tannin and colouring matter are retained on the filter in combination with magnesia, while the organic salts of magnesia, which are mostly soluble, pass through with the filtrate. 10-30c.c.of the filtrate, according to the amount of acid present, is evaporated to dryness, and gently ignited so as not to decompose any magnesic sulphate present. The residue is moistened with water saturated with carbonic acid, to convert any magnesic oxide into carbonate, and then dried, in order to make the mass powdery, and easier to wash, It is next taken up with hot distilled water, filtered and well washed. Any sulphate which is present passes into the filtrate, while the carbonate, which corresponds to the organic salts present before ignition, remains on the filter, and after solution in hydrochloric acid, is estimated as magnesic pyrophosphate. To the hydrochloric solution is added excess of ammonia and sufficient ammonic chloride to redissolve the precipitate formed, and prevent the precipitation of the magnesia; the solution is heated and then ammonic oxalate solution, first dilute, and then concentrated, is added to precipitateany lime which may be derived from lime salts present in the liquor. After filtering out and washing the precipitate, 10-15c.c.of 10 per cent. sodic phosphate solution is added, and the liquid is stirred with a glass rod without touching the sides of the beaker, and allowed to stand 12 hours. The crystalline precipitate is then rinsed on to a filter, and washed with a mixture of 1 of ammonia and 3 of water, till the washings no longer give any milkiness with silver nitrate. The filter is then dried and the precipitate is placed in a platinum crucible and first gently, and then strongly ignited with the cover on; the filter paper, freed as much as possible from the precipitate, is burnt in the usual way on the crucible lid, the ashes are added to the precipitate in the crucible, and the whole is again ignited and allowed to cool in the desiccator, and finally weighed. 111 parts of magnesia pyrophosphate correspond to 120 parts of acetic, or 180 parts of lactic acid. Kohnstein and Simand calculate the pyrophosphate corresponding to the acetic acid already found by distillation, and after deducting it reckon out the remainder as lactic acid. Of course the volatile acids are really a mixture consisting of acetic, propionic, butyric and other members of the fatty group; but it would be difficult if not impossible to separate them. Similarly other fixed acids exist in mixture beside the lactic acid, but as their action is similar and lactic acid is always the most abundant, these acids are to be reckoned as lactic.
It has been mentioned that when sulphuric acid is present in the liquor it is found in the filtrate from the magnesia carbonate as sulphate. After removal of the lime as oxalate, as previously described, the magnesia may be similarly determined as pyrophosphate, and reckoned out as sulphuric acid (111 parts of pyrophosphate being equal to 98 parts sulphuric acid, H2SO4). It may also be estimated with barium chloride, but in this case regard must be had to the sulphates originally present in the liquor.
Since waters invariably contain both lime and magnesia salts, a portion (50 or 100c.c.) must be evaporated, ignited,and after precipitation of the lime, the magnesia must be estimated as already described, and deducted from the amount found in a similar amount of liquor after saturating with magnesia. If, together with the organic acids, the liquor contains sulphuric acid, the correction may be divided equally between the two.
The method is not applicable in presence of phosphoric, tartaric, or oxalic acids. To overcome this difficulty, Messrs. Kohnstein and Simand are at present investigating a method dependent on decolorisation of the liquor with bone charcoal, completely free from mineral salts, and subsequent titration with soda.
It may be interesting to add the determinations of a complete set of handlers in a Continental upper-leather tannery, in which larch bark is used. 100c.c.of liquor contained as follows, ingrm.:—
QUALITATIVE DETECTION OF TANNINS.
It is often desirable to determine from what tanning materials an extract or liquor is made, or with what a sample of leather is tanned. The following table gives reactions of the principal tanning materials, which will enable any one of them to be recognised with certainty, and in many cases will determine the constituents in a mixture of several, though this is naturally far more difficult. In such cases, colour reactions are apt to mislead, that of one tannin being modified by another, and it is safest to rely on the categorical test of precipitate or no precipitate, coloration or no coloration, without regard to the tint. The infusions must be very weak, not exceeding 1-2° Bktr., or precipitates will be formed where mere coloration or clouding is noted. In some cases only negative peculiarities can be given, and the material cannot be positively determined in mixture with materials where these peculiarities are present. Thus myrobalans could not be distinguished from divi with certainty, where any other material, such as gambier, was present, which gave a deep coloration with concentrated sulphuric acid. The writer will feel greatly obliged by the communication of more distinctive reactions.
CHEMICAL ANALYSIS FOR THE TANNERY
CHEMICAL ANALYSIS FOR THE TANNERY — Continued.
QUANTITATIVE DETERMINATION.
Many processes have been proposed for the quantitative estimation of tannins, but it cannot be said that any method yet known is wholly satisfactory. The oldest, that of Sir H. Davy, recently improved by Stoddart and others, consists in precipitating with gelatin, and drying and weighing the precipitate. This is almost impossible to filter off as directed by Davy; but by the use of a little alum, and by pouring hot water on the precipitate, it becomes curdled into a mass which may be washed by decantation. As the precipitate contains varying quantities of tannin, according to the strength of solution employed; as it is soluble in excess of gelatin solution, and as it is almost if not quite impossible to wash it free from gelatin and alum, the method can hardly lay claim to much accuracy. A somewhat better one consists in the employment of a standard solution of gelatin with a little alum, determining the end of the reaction by filtering off a portion and ascertaining if another drop of the reagent produces a further precipitate. This method is very tedious, the end reaction is difficult to hit, the standard solution is very unstable, it is inapplicable to gambier and cutch because themixture will not filter clear, and its results are irregular, probably from the power of tannin to combine with various proportions of gelatin. A plan, which has a seductive appearance of simplicity, is that of Hammer; he takes the sp. gr. of the infusion, then absorbs the tannin with slightly moistened hide-raspings, again takes the sp. gr., and from the difference calculates the percentage of tannin, a difference of 5 per cent. of tannin corresponding to one of 1·020 sp. gr. (20° barkometer). Unfortunately the hide is more or less soluble in the liquor, and absorbs acids other than tannic with considerable energy; the moistening of the raspings introduces an error, and the smallness of the quantity to be measured makes a slight error completely vitiate the results. With extreme care, due corrections for temperature, for the water introduced with the raspings, and for their solubility, and by substituting evaporation of the infusions to dryness for mere calculation from their sp. gr., the method is useful as giving almost the only information obtainable as to the actual weight of tannin in any material capable of being absorbed by hide. It is, however, only suitable for use as a check on easier and more rapid methods, such as Löwenthal's, which give accurate relative results, but no information as to absolute weight of unknown tannins. A modification of Hammer's method has been introduced by Müntz and Ramspacher, in which the liquor whence the tannin is to be removed is forced through a piece of raw hide by pressure. This method, except that it is more rapid, has all the evils of Hammer's in an intensified form, and gives such variable results as to be quite useless in practice. A set of very careful determinations of one sample of sumach gave results ranging from 18 to 28 per cent., and similar variations occurred when the experiment was repeated with valonia. Wagner's method by precipitation with a standard solution of cinchonine and magenta has proved wholly unreliable.
Gerland's method with a volumetric solution of tartar emetic, used in presence of ammonic chloride, gives constant results with sumachs,2/3of those given by permanganate andNeubauer's equivalent. Tartar emetic does not precipitate the tannins of cutch and gambier. Fleck's, by precipitation with copper acetate, and subsequent washing with ammonic carbonate and gravimetric estimation, either of the tannate dried at 212° F. (100° C.), or of the copper oxide left on ignition; and Carpene's, by precipitation with ammoniacal zinc acetate, and subsequent estimation with permanganate and indigo, though giving fairly accurate results on some tannins, are only of limited application. They may therefore be passed over, as well as Jean's method with a volumetric solution of iodine in presence of sodic carbonate, and Allen's method with lead acetate, which are tedious and difficult, and present no advantage over Löwenthal's improved process. This last is easy of execution, constant in results, and universally applicable. Before proceeding to describe it in detail, it may be well to give some hints as to the best modes of sampling and preparing tanning materials for analysis, since this is often more difficult and tedious than the actual analysis.
Sampling.—Samples should always be drawn from at least 10 sacks or separate parts of the bulk, and, in the case of valonia, special care should be taken to have a fair average quantity of "beard." No attention is usually paid to this point by merchants, and the proportion varies greatly in different parts of the same cargo. If several sacks are spread in layers on a level floor, and then portions going quite to the ground are taken from several parts of the floor, this will be accomplished. Where samples must be dealt with which have not been specially drawn, it might be safest to weigh out from each the same proportion of beard and whole cups, bearing in mind that the beard is always the richest part of the valonia. In sampling myrobalans, it should be remembered that the poor and light nuts will rise to the top, and hence the hand should be plunged well into the sack. Grinding of valonia and myrobalans when practicable is probably best done in a small disintegrator, fitted with gratings. The material, of which some pounds must beused, is screened over a sieve of say 15 wires per in., and all coarser parts are returned to the mill till they will pass. The mill must grind into a close box, that no dust may be lost. Bark may be reduced to fine saw-dust by cutting a portion of each piece in the sample with a circular saw or rasp driven by a lathe. The advantage of these methods is that samples can be ground without previous drying, and thus in many cases time may be saved and separate determination of moisture avoided. When this is not practicable, the sample of some lbs. at least is ground in an ordinary bark-mill, well mixed, spread out flat on a floor or table, and several portions are taken as already described, say 50-100grm.in all, and dried in a water- or air-oven at 212° F. (100° C.). The moisture is best determined, to save time, in a small separate portion of 10grm., which must be dried till it ceases to lose weight, and the loss taken as moisture. It must be weighed in a covered capsule, as it is very hygroscopic. When the larger portion of the sample has been dried some hours, it is passed twice through a good coffee-mill, and then returned to the oven till thoroughly dried, for which, 12-24 hours is generally sufficient. Another method sometimes convenient is to take each acorn, or each piece of bark of the sample to be tested, and snip a piece from it with a pair of tinners' shears, taking care that in the case of valonia the section runs right to the centre of the cup; and in bark, that fair shares of the outer and inner layers are taken. The reason for drying before grinding is, that unless hard dried, tanning materials cannot be passed through a small mill. Bark and valonia usually contain 12-16 per cent. of moisture.
Exhaustion.—10grm.of valonia, 20-30grm.of bark, or corresponding quantities of other material, are boiled briskly for half an hour with 1litreof distilled water, a funnel being placed in the neck of the flask, and great care being taken at first to avoid frothing and boiling over. The flasks used should have a capacity of at least 11/2litre. The whole contents are finally rinsed into a gauged flask, allowed to cool to 59° F. (15° C.), and made up to 1litre. In the case ofsumach, a little more boiling even than this is desirable. This method has been found by the writer to give better results than boiling with successive portions of water. Another method is to boil for1/2hour with 250c.c.of water, then pour the whole on a filter, wash with boiling water so long as a drop of the filtrate blackens paper moistened with a dilute solution of ferric acetate, and finally make up to 1litre. Many materials, however, clog the filter to such an extent that washing is almost impossible. Kathreiner has used 15litresof water, and corresponding quantities of material, in a large steam-jacketed copper pan, for each exhaustion, making the weight up finally to 15kilos., with very uniform and excellent results. (See alsop. 130.) With all materials which deposit ellagic acid or other insoluble derivatives, on cooling and standing, considerably higher results will be obtained if the titration be made as soon as the liquor is cold, than if it be allowed to stand 24 hours; in this respect, a uniform practice should be adhered to. Addition of1/2c.c.of glacial acetic acid renders the infusions less liable to change.
Analysis.—Of all the methods which have been proposed for the estimation of tannins, the only one which has met with any general acceptance is that of Löwenthal, and indeed it is the only one which in rapidity of execution and constancy of results is fitted for general use. The method, as originally proposed, depends on the oxidation of the astringent solution by permanganate in presence of indigo, which not only serves as an indicator, but controls the oxidation, limiting it to those bodies which are more oxidisable than indigo. As, however, these include gallic acid and other substances which are useless to either tanner or dyer, it is necessary to remove the tannin, and by a second titration to obtain its value by difference. This Löwenthal (Zeitschrift f. Anal. Chemie, 1877, p. 33) accomplished by a solution of gelatin and common salt, to which, after mixture with the tannin infusion, a small quantity of sulphuric or hydrochloric acid was added. It was necessary to let this standat least some hours before a clear filtrate could be obtained, and the gelatin remaining in solution had a slight though generally negligible effect on the permanganate. In some cases, even after long standing, perfect filtration was extremely difficult and tedious, and it was also clearly proved by Simand (Ding. Polyt. Jour., ccxliv. 400) that a certain proportion of the tanno-gelatin precipitate, varying with the acid present, and with the species of tannin, remained in solution, and thus gave too low a result. He therefore proposed to revert to the old method of separating tannin with hide raspings, or, as an improved substitute, with the gelatinous tissue of bones, and this is probably the most accurate method, but has the disadvantage of requiring considerable time for its execution. (See alsop. 130.) The writer has therefore tried, and he thinks successfully, so to modify Löwenthal's original method as to increase its accuracy, and at the same time to make it more rapid and easy of execution. It was found that by saturating the clear filtrate with salt, a further precipitate containing tannin was formed, but unfortunately, it was so finely divided that no amount of standing, or even of warming, and repeated passing through the paper, would obtain a clear filtrate. Finally, he hit on the device of mixing with the liquid, before filtration, a portion of the pure kaolin used by photographers. The effect was instantaneous and complete. A perfectly clear filtrate was obtained without any of the tedious waiting which before was necessary, and it was not only free from tannin, but also nearly so from gelatin, so that it only gave the faintest cloudiness with tannin solution. Gelatin gives a more considerable precipitate, but this is simply due to its insolubility in the saturated salt solution, and it is redissolved on dilution with water.[L]