glass tubing, with an oxygen reservoir. This combustion tube is filled half way from the bent end with granulated cupric oxide, which may conveniently be held in position either by plugs of asbestos or by platinum wire gauze, or by a combination of both. The connection with the oxygen reservoir being then made, the greater part of the tube is heated to redness, with the ordinary precautions, and a stream of oxygen (which is first conducted through a long tube containing caustic potash) is passed over the glowing oxide of copper until the issuing gas ceases, after long bubbling, to cause any turbidity in the bright baryta water. As soon as this point is reached, the portion of the combustion tube preceding the layer of cupric oxide is allowed to cool somewhat, and the tube is now ready to be connected with the absorption apparatus. The clean absorption tube is carefully rinsed with water, and is clamped in front of the furnace in such a manner that its bulb end is somewhat higher than the end to be connected to the combustion tube. Both ends must be provided with convenient stoppers, consisting of short pieces of caoutchouc tubing closed with a small piece of glass rod. The stoppers being removed, air, which is first caused to pass through a tube containing caustic potash, is pumped through the tube for about two minutes, and it is then filled with baryta water as follows: The baryta water (of strength 1.5 per cent.) is kept in a sufficiently large stock bottle, provided with a caoutchouc stopper, through which pass two bent glass tubes, the long one for syphoning, the shorter, to which a potash tube is attached, being connected with a small hand-bellows. In filling the absorption apparatus, the longer syphon tube is connected with it by means of flexible tubing, and the baryta water is forced over by gentle pressure of the bellows, the bulb end of the absorption apparatus being provided with a potash tube. As soon as the absorption apparatus is half filled, the flow of baryta water is arrested; the ends of the Pettenkofer tube are immediately closed by its stoppers, and it is now ready for use. By these means the tube is filled with perfectly clear and bright baryta water. The absorption apparatus is now connected with the combustion tube, and the combustion proceeded with. The silver dish containing the water residue having been inserted just behind the copper oxide, it is burnt in a slow current of oxygen, and the carbon dioxide is absorbed and converted into baric carbonate in the absorption tube. In order to filter off and convert the baric carbonate, a funnel and filter are arranged to stand over a beaker containing a layer of caustic potash solution at the bottom, the whole being covered by a bell jar, which itself stands in a layer of caustic potash solution. The mouth of the bell jar, which is immediately over the funnel, is closed by a thick caoutchouc cap with two narrow openings, one of which is provided with a caustic potash tube. (Soda lime apparently answers equally well.) The other, which is temporarily stoppered, contains a straight glass tube, placed immediately over the filter so that, after the whole arrangement has been left some time to itself, in order that all enclosed air may be free from CO2, direct connection may be made with the Pettenkofer tube by means of flexible tubing sufficiently long to admit of some slight freedom of action. Filtration may thus be carried on without danger of CO, being introduced from the atmosphere, the additional precaution being taken of compelling all air which passes through the Pettenkofer tube during this process of filtration, to pass through a tube containing caustic potash attached to the tube itself. The washing of the precipitate in the tube and on the filter is effected almost entirely with boiling water, which has been previously saturated with carbonate of barium [solubility 1 in 15,000], but finally with a small quantity of boiling distilled water. After complete washing, the tube is disconnected, and the filter ultimately rinsed round, while still under the bell jar, by means of the long tube already mentioned, and which, when not clamped, may be moved freely in all directions. The bell jar is then removed, and the precipitate is rapidly washed together into the bottom of the filter. The Pettenkofer tube, which may contain minute particles of baric carbonate not removed by the washing, is rinsed twice with small quantities of dilute pure hydrochloric acid (about 1 in 50), and finally with distilled water: the rinsings are poured on to the filter on which the greater mass of baric carbonate is already collected. The filter is further washed with dilute hydrochloric acid, and finally with distilled water and the whole of the solution of baric chloride so formed is carefully collected in a small beaker. The quantity of such solution need not exceed 50 cc. This solution of chloride of barium has next to be evaporated, which is best done in a platinum vessel on the water-bath. It is then transferred, when greatly decreased in bulk, to a much smaller platinum dish, weighing about 5 grms., and finally evaporated to dryness after the addition of a few drops of pure sulphuric acid. The dish and its contents have then to be ignited, the residue moistened with a drop of nitric acid and redried, and the whole re-ignited and weighed to conclude the operation. The amount of carbon present is obtained by dividing the weight of the baric sulphate by 19.4. Nephalometric Method. This ingenious method we also owe to Dupré and Hake. The carbonic acid resulting from the combustion of an organic residue is passed into perfectly pure clear solution of basic lead acetate, and the turbidity produced is imitated by known weights of CO,; in fact, the operation is a colour method conducted on the same principle as "Nesslerising,” with this important difference, that no success will be obtained unless there are special precautions taken to prevent the contamination of the solutions by the breath and air, &c. § 319a. The Estimation of Organic Nitrogen after Kjeldahl's Method.-Half a litre of the water is placed in a retort and the free ammonia distilled off. Then 5 to 10 cc. of diluted sulphuric acid are added, and the water concentrated down on a sandbath until the acid fumes. The acid is allowed to fume for about half an hour, or until it is almost colourless. The acid solution is then cooled, diluted, alkalised with pure soda lye, and the liquid distilled, the alkaline distillate being neutralised with decinormal sulphuric acid, each cc. of which is equal to 14 mgrm. of nitrogen. This simple process is applicable to most pure waters containing but little organic matter and feeble nitrates. On the other hand, it will not give accurate results with waters containing much nitrate or much organic matter. In such a case the following is the best method :*-Half a litre, as before, is taken. The water (after getting rid of the free ammonia) is saturated with SO2, and a drop of iron chloride solution added; it is then gently heated for about twenty minutes; and is next boiled down to about 20 cc. To this residue is added 20 cc. of sulphuric acid containing 4 grms. of phosphorus pentoxide and then 0.12 grm. of anhydrous copper sulphate and 5 drops of platin chloride solution. The contents of the flask, closed by a glass marble, are heated gradually to a gentle boil, and the heating continued until the fluid remains of a green colour. After cooling the acid fluid is diluted, alkalised by ammonia-free soda solution, a little granulated zinc added, and the whole distilled into a measured volume of decinormal sulphuric acid. The decinormal acid arrests any ammonia, and on titrating the distillate with d. n. soda, there will be a loss of acidity proportionate to the ammonia, from which (as before) the amount of nitrogen can be calculated. There has been found a slight practical difficulty in thus titrating ammonia with the greatest accuracy, and Kjeldahl† has, therefore, recommended the utilisation of the reaction which takes place between a free acid and iodide and iodate. This reaction (denoting free acid by HR) takes place as follows: 6HR + 5KI + KIO = 6KR + 3H2O + 61. To the acid distillate is added 0-4 grm. of potassium iodide * Ulsch, Zeitschrift f. analyt. Chemie., xxv. 579. and 0.1 grm. of potassic iodate; after standing two hours the iodine set free is estimated by means of a decinormal thiosulphate solution, the strength of which has been checked and adjusted by the aid of a decinormal iodine solution. An example will make the calculation clear. The distillate from half a litre of water was submitted to Kjeldahl's process, and was received in 30 cc. decinormal sulphuric acid. Potassic iodide and iodate were added thereto (as above described), and the mixture allowed to stand two hours; at the end of that time the free iodine was estimated by means of a decinormal thiosulphate solution, using starch as an indicator; 259 cc. of thiosulphate were used. Since 30 cc. of thiosulphate are equivalent to 30 cc. of decinormal acid, it is clear that 30 25.9—that is, 4·1 cc. of free acid-have been saturated with ammonia; hence the distillate contains 4.1 cc. x 1.4 mgrm. 5.74 mgrms. nitrogen, and the water contains 11.48 mgrms. per litre or 11:48 parts per million of organic nitrogen. = (12.) Mineral Analysis of Water.*-Ordinary drinking water holds dissolved but few saline matters, and when an analyst has determined chlorine, nitrates, sulphates, phosphates, and carbonates, and also lime and magnesia and alkalies, he will usually find, on adding the several amounts together, that he gets numbers very nearly equal to the solid saline residue. An excellent method of approximately estimating the various saline constituents of a water is to evaporate down to dryness a known quantity, then to treat the residue with a little hot water, which will dissolve all the *A method of determining calcium and magnesium has been described by Professor C. L. Bloxam (Chem. News, 1886); it depends upon the precipitation of calcium and magnesium as ammonio-arsenates, and is specially applicable to their separation from strontium salts. The determination of calcium as ammonio-arsenate in ordinary drinking waters has some advantage-the precipitate is as nearly insoluble in water as calcium oxalate, and being highly crystalline is not liable to run through the filterthe formula of the precipitate dried at 100° is Ca¿NH4H¿(AsO4)6.3H2O, and 100 parts are equal to 20 of calcium or 50 calcio-carbonate. If a rapid determination be desired, arsenic acid is added to 4 of a litre in water, which is then strongly alkalised by ammonia. The mixture is well stirred, and allowed to stand for ten minutes, the precipitate is then collected on a weighed filter, washed with ammonia water (8.5 per cent.), and dried at 100°. The gain in weight represents the united magnesic and calcic ammonioarsenates. The precipitates are now dissolved off the filter by acetic acid, and the calcium precipitated as oxalate by ammonium oxalate the solution boiled and filtered. On adding to the filtrate ammonium, the ammonio. magnesium arsenate is in this way reprecipitated, and may be collected on a weighed filter, washed with ammonia water, and dried at 100°; its composition is (MgNH4AsO4}H2O); 100 parts are equal to 44.2 of magne sium carbonate. soluble salts out, but leave insoluble carbonates of lime and magnesia, and silica. In the soluble portion, the soluble salts of the alkaline earths and the alkalies are determined; the chlorides, sulphates, and nitrates are estimated on the unconcentrated water by the processes already detailed. It is also always open to make the analysis in the old-fashioned waythat is, to evaporate down a large quantity of water, to separate the silica by treatment of the ash or residue with hydrochloric acid, and after separation of the silica to divide the solution into three or four quantities, in which sulphuric acid, lime, magnesia, &c., are determined by the ordinary methods. T IV. BIOLOGICAL METHODS. § 320. A. Microscopical Appearances. To make a microscopical examination of water, it is necessary to collect the sediment or deposit which falls to the bottom of the vessel in which the water stands. A convenient way of doing this is to use the author's tube (fig. 77), which holds a little more than a litre. The little glass cell, C, is adjusted to the pipette-like end, the rod is removed, and after introduction of the water the tube is covered and set aside for twenty-four hours. At the end of that time any deposit will have collected in the glass cap. On now carefully inserting the rod-like stopper, the cap or cell can be removed with great ease, and its contents submitted to microscopical examination. With very pure waters merely a little sand or formless débris collects in the cap, and there is no life. If, however, in the first place eight or ten gallons are allowed to deposit in a capacious vessel, most of the water run off, and then the last litre rinsed into the tube, in nearly every case there may be a few life-forms and sufficient matter collected to give definite results. It need scarcely be said that an opinion must not be formed upon a microscopical examination without taking into account the amount of water from which the sediment has been collected, and a definite quantity should be generally Fig. 77. agreed on by analysts. C The Sedgwick-Rafter Method.*-This ingenious method of obtaining a quantitative estimation of organisms and objects in water is in use by the Massachusetts biologists. A brass gauze stop is put in the mouth of a funnel, and on this stop is packed a layer of sharp quartz sand; the sand grain should pass through a sieve 80 meshes to the inch, but not through a sieve 100 to the inch. On to this sand is poured a con * Massachusetts State Board of Health Report. Boston, 1890. |