Imagini ale paginilor
PDF
ePub

rated from the mother liquors containing the liquid tar acid and a residue of carbolic acid dissolved in them. Complete purification of the carbolic-acid crystals thus obtained is effected by recrystallization, either by partial fusion or solution in water, with subsequent refrigeration. Carbolic acid of high degrees of purity is obtained by dehydrating these carbolic-acid crystals.- Practical Magazine, Dec., 1874, 437.

DETECTING ADULTERATIONS IN COFFEE. Wittstein, in a paper published in “Dingler's Polytechnic Journal,” gives a detailed method for testing coffee, which is treated in a great variety of ways for the purpose of imparting greater strength or weight, a better color, or some other desired quality. He remarks that roasting, and still more grinding, coffee renders it almost impossible to apply the necessary tests. The principal vegetable substances used in adulterating coffee are chicory, beet-root, carrot, figs, corn, malt, etc. A simple method of testing coffee for a mixture of chicory is to shake it with water; if pure it remains floating for an hour together, whereas chicory sinks. An infasion of burned chicory, diluted with much water and treated with potassium bichromate, undergoes no visible change; but coffee assumes a deep brown tint, causing a resemblance to porter. This is only observed, however, where the substance is either entirely coffee or entirely chicory. To detect a mixture of the two, the decoction is diluted with eight times its bulk of water, filtered, and the dilution increased to twelve parts. If the decoction contains pure coffee, on adding to thirty drops of it two drops of concentrated bydrochloric acid, boiling for a few seconds, then adding fifteen drops of a solution of one part of potassium ferrocyanide in eight parts of water, and boiling as before, the solution becomes first green, then dark green. Six drops of potash are next added, and the whole is boiled for two minutes, the solution becoming first brown and then clear pale yellow, with a slight dirty-yellow precipitate. With chicory alone, the solution finally remains brown and turbid, and after long standing a precipitate falls, the supernatant fluid retaining its brown color. With a mixture of twenty-four drops of coffee and six of the chicory decoction, a final brown turbid solution is obtained. A decoction of coffee of average strength contains one per cent., and of great strength barely two per cent., of the dry soluble matter. The evaporation residue consists of a deep brown, shining, varnish-like mass, feeling quite dry to the touch after two days. If the ordinary substitutes are present, however, this residue becomes sticky after one or two hours' standing, and quite damp in twenty-four hours.

ON THE GASES INCLOSED IN COAL. An examination of the gaseous substances contained in coal, and also of the gases which are evolved from fissures in coal-mines, has been made by Thomas. The coals which he examined were of three sorts: the bituminous or house coals, the semi-bituminous or steam coals, and the anthracite coals. The samples examined all came from the South Wales coalbasin. In obtaining the gas from the coal, slices were first sawn from the middle of large cubical blocks, and a strip cut from the centre of each of these six or eight inches long, and five eighths of an inch square. This was then placed in a hard glass tube, connected with a mercury pump, and exhausted. Very little gas, however, was obtained in this wayonly two or three cubic centimeters in 100 grammes of coal. On placing the tube in boiling water, the exhaustion being continued, gas was freely evolved. When it ceased, the temperature was raised to 200° and even 300° Centigrade, whereby additional quantities of gas were obtained. On analyzing the gas, it was found to be composed of marsh gas, carbon dioxide, nitrogen, and oxygen. The bituminous coals give the minimum amount of gas, but the quantity of carbon diox. ide contained in it is large. Steam coals come next, as much as eighty-seven per cent. of the gas obtained from them being methyl hydride, or marsh gas. Anthracite coals give more gas than the others, one specimen giving 600 cubic centimeters of from 100 grammes of coal when heated to 100°. But at 200°, 1000 c. c. were given off, and at 300°, 1875 C. C. In composition, the gas from anthracite closely resembles that from steam coal. The examination of the gas collected from fissures in the mine, as well as that obtained by boring the blocks of coal, was almost pure marsh gas, the amount present rising sometimes even to 97.65 per cent.--21 A, II., xiii., September, 1875.

gas

[ocr errors]

SCHÖNBEIN'S TEST FOR NITRATES. Storer has explained at length the reaction proposed by Schönbein for the detection of minute traces of nitrates.

Two modes of applying the test were originally given. In • the one, dilute sulphuric acid and iodo-starch paste were

added directly to the nitrate solution, and the mixture was stirred with a zinc rod. In the other and better mode, the nitrate was first reduced by means of zinc or cadmium, and then the solution was acidulated with sulphuric acid and the iodo-starch paste added. He finds, contrary to the opinion of Carius, that no objection seems to lie against this test on the score of delicacy; but that it has a fatal defect in the forms in which it is commonly used, in that mere water, entirely free from any nitrates or nitrites, will, on being treated with zinc or cadmium as in the process of testing, react upon iodo-starch as if these compounds were present. This coloration, thus produced, the author shows is due to the hydrogen peroxide which is formed by the action of the metal itself upon

the water- a reaction observed by Schönbein himself. Hence, whenever the degree of coloration of the iodo-starch obtained in testing for a nitrate by this method is less intense than the tint obtainable from 0.000187 gramme of potassium nitrate in 50 cubic centimeters of water, it is difficult to decide whether the coloration may not be wholly due to hydrogen peroxide. Having thus shown the defect, the author set himself at work to remedy it, and to devise a mod. ification of the process which, while preserving the delicacy of the test, should yet be easily applied. The very simple plan was adopted of acidulating the solution very slightly

, before boiling the nitrate in it, with metallic cadmium. Under these circumstances no hydrogen peroxide is produced, while the reduction of the nitrates goes on quite as well. The only precaution necessary is to prevent the loss of any of the nitrous acid, which is easily accomplished by attaching to the flask a small inverted Liebig condenser during the boiling. In his experiments, 0.0001 gramme of nitrogen pentoxide, placed in the solution as potassium nitrate, in 50 c. c. of water containing two drops of dilute sulphuric acid, gave, after boiling for five minutes, a reaction in less than fif teen minutes. Even 0.00005 gramme gave the reaction in

the course of half an hour. Zinc, amalgamated zinc, aluminum, iron, lead, and magnesium were also used as reducing agents, but none of them acted as efficiently as cadmium. No experiments were made with the alkali metals.—4 D, III., xii., September, 1876, 176.

HYDROCELLULOSE. In some of the processes in the arts in which woody fibre is used, as in paper-making, for example, the fibre appears to undergo during the process of manufacture a peculiar transformation, by which it is rendered friable. Aimé Girard has investigated the matter, and finds that this change is owing to the assumption of a molecule of water by each molecule of the cellulose, thus producing a new body having the composition C2H,2011, to which he gives the name of hydrocellulose. To prepare it some form of purified cellulose, such as carded cotton, is placed in sulphuric acid of 45° Baumé in the cold for twelve hours. It is then well washed, pressed, and dried. After it is dry, its fibrous character is destroyed by pressure; rubbing between the fingers converts it into a white powder. Girard supposes that this substance may be formed in the process of bleaching paper-pulp by chloride of lime; and accounts in this way for the peculiar brittleness of certain

papers found in commerce.-—6 B, LXXXI., December, 1875, 1105.

A NEW OXIDE OF SULPHUR. Weber has investigated the cause of the intense blue color which is developed whenever sulphur is allowed to act upon sulphuric oxide or disulphuric acid, and has shown that it is due to an oxide of sulphur hitherto unknown, which he has succeeded in isolating and examining. To prepare it, carefully dried flowers of sulphur are thrown in small portions at a time into sulphuric oxide containing sulphuric acid. At the instant of contact, the sulphur is converted into dark blue liquid drops which sink to the bottom of the liquid and there solidify. Care should be taken to keep the temperature at 15° Centigrade, since below this point the whole liquid solidifies, and above it the blue body decomposes. After the operation the excess of liquid is poured off, the blue crystalline crusts are drained, and the excess of sulphuric oxide driven off at a temperature not exceeding a blood heat. Bluishgreen crusts are thus obtained which are very friable, and which have a structure similar to malachite. They decom. pose without fusion slowly at ordinary temperatures, more rapidly on heating, evolving sulphurous oxide, and leaving sulphur behind. In a cool place the decomposition is so slow that the substance may readily be weighed for analysis. Moist air decomposes it rapidly, and it hisses when thrown into water. Alcohol and ether also decompose it and set free sulphur. A mean of five closely accordant analyses showed that it contained 57,12 per cent. of sulphur, thus giving it the formula S, 03. The author names it sulphur sesquioxide, or dithionic oxide. No compounds of it have yet been made. Selenium gives an analogous compound, having the formula SeSO3. It is dirty green in mass, but is yellow when in powder.- Poggendorff's Annalen, CLVI., December, 1875, 531.

CORROSION OF PLATINUM STILLS BY SULPHURIC ACID. Scheurer-Kestner having communicated to Hofmann, in 1862, certain results which he had obtained in the process of concentrating sulphuric acid in platinum stills (which results were published by the latter in his Report on Chemical Industry), and these results having been since that time called in question, the author has examined the facts in the case still more fully, and now publishes a new set of observations. From 1851 to 1861, 4309 tons of sulphuric acid were concentrated to 66° Baumé, in an alembio the body of which weighed 40 kilogrammes. The entire loss of this part of the still during this time was 12,295 grammes, being 2.859 grammes for each ton of acid worked. Perceiving that the cause of this large loss in platinum was the presence of nitrous products, ammonium sulphate was added to the acid in amount just sufficient to destroy them. In 1862, 1843 tons of acid were concentrated in the still, with a loss of 2490 grammes, being only 1.22 grammes of platinum for each ton of acid, a marked decrease. From 1864 to 1875, 17,516 tons (of 1000 kilogrammes each) were concentrated to 66° in still the body of which weighed 50 kilogrammes. The acid contained sulphurous acid, but no nitrous compounds. The loss of the still was 16,178 grammes, or 0.925 gramme to the

« ÎnapoiContinuați »