site each other in the terminal circular planes. By means of well-fitting perforated corks a funnel tube of tin-plate or glass is inserted into the distant horizontal aperture, and an exit tube of tin or glass into the other, while the distilling tube extends through the axis of the cylinder. The hot contents of the distilling tube pass downwards, becoming colder and colder in their descent, while the stream of cold water entering the condenser through the funnel passes upwards, becoming hotter and hotter in its ascent until it escapes at the overflow pipe. The condenser may be fastened to a retort-stand with string or wire, as in the figure, or may be supported by a clamp of some kind. Liebig's condensers are made in every variety of form, size, material, and construction, some of them being provided with special supports, which allow them to be heightened or lowered at will, and placed any desired inclination. at (21.) Heat is applied to solids in order to warm, dry, ignite, fuse, or volatilise them. Any tube, flask, or retort, the interior of which cannot be reached by the fingers, should, after thorough cleaning,* be rinsed once or twice with distilled water, and then drained as dry as possible by means of draining pegs or some other mode of support. It should next be warmed carefully over a gas flame or in front of a fire, and the hot moist air sucked out of it from time to time by the aid of a long tube reaching into it for some considerable distance, as shown in fig. 26. Narrow glass tubing is Fig. 26. There is seldom much difficulty in cleaning laboratory glass from any stain or dirtiness, when it is not of long standing, by means of cold or hot water and dilute or strong acids or alkalis, aided by, extempore brushes made of moist tow dipped in sand and fastened on to thick pieces of wire, or by tube-brushes made specially for the purpose. Flasks, retorts, &c., may often be very efficiently cleaned by shaking them up somewhat violently, after the introduction of a little water and a few pieces of soft paper or rag. The interior of narrow glass tubing is best cleaned by pushing a piece of moist filtering paper through it. dried by heating some length of it over a gas burner or spirit lamp, and simultaneously sucking air through it with the mouth. In the absence of an air- or water-oven, reduction tubes and similar small pieces of apparatus may be dried by heating them on a sand-bath standing over a burner; or preferably on a flat iron plate, which in many other cases also may be advantageously substituted for a sand-bath. Moist powders of various kinds may be dried on a water-bath, or sand-bath, or by ignition over an argand burner, &c., according to circumstances. A washed precipitate retained in its filter and funnel may often be quickly Fig. 27. dried by supporting the funnel on a broken beaker or short lamp-glass standing upon a trellis of iron wire, underneath which a small gas-flame is kept burning (fig. 27). Or the filter may be supported over a heated iron plate by means of a small tripod stand, easily made out of copper-wire. When nearly dry, the filter with its contents may be removed from the funnel and placed in a water-bath; or the washed precipitate and filter may be removed carefully from the funnel, pressed gently between folds of blotting-paper, and placed at once in the water-bath. At moderate temperatures, drying over oil of vitriol in the exhausted receiver of an air-pump takes place with considerable rapidity. Moreover, a shallow air-pump-jar standing on a plate of ground glass over a dish of oil of vitriol or quicklime, forms a very convenient chamber in which all sorts of bodies may be dried and kept dry. Animal solids are frequently subjected to ignition in order to burn off their organic, and leave behind their mineral matter, or ash. The tissue, &c., may be first carbonised in small portions at a time in a thin Berlin capsule or crucible, heated over a gauze burner, in some place where the empyreumatic vapour, &c., can be readily got rid of. The resulting charcoal should then be pulverised, and the powder heated steadily for some hours in a shallow platinum capsule, or on a tray of platinum foil, supported over an argand flame, when the charcoal will gradually burn away, and a white or greyish ash be left. The temperature should never exceed that of dull redness, as otherwise the ash, save that of blood, is apt to fuse over the remaining charcoal, and so prevent its combustion. The operation is much facilitated by protecting the capsule from draughts, and particularly by placing over, but not immediately upon it, a cover of thin platinum foil. Carbonate of sodium and other fluxes employed in testing, are often heated to dull redness over an argand flame just before being used. Moreover, in quantitative analysis, filters and their contained precipitates have constantly to be burnt, with a view to getting rid of the filter-paper and leaving the precipitate in a state fit for being weighed. The ignitions made on charcoal or platinum wire in the course of blowpipe testing will be presently described. Independently of the many fusions made in the course of blowpipe testing, others on a somewhat larger, though still very small scale, have occasionally to be performed by the student. There are, for instance, a few substances which require to be fused with carbonate of sodium or potassium, either alone or mixed with some other reagent, before they can be brought into a state of solution, and so identified by ordinary analytical processes. The insoluble substance is usually incorporated with three or four times its bulk of a mixture of carbonate of sodium with carbonate of potassium, or in some cases with either the nitrate or cyanide of potassium, and heated to thorough fusion over a Bunsen or blowpipe flame, in a platinum or porcelain capsule, or in a small iron spoon. In making these fusions, it is most important that both the substance and flux be well dried, very finely powdered, and intimately commixed. The capsule or crucible should be heated at first very gradually, but ultimately to the highest attainable temperature. The only volatilisations which the student will be called upon to perform are made in narrow glass tubes,-open at both ends when a current of air is required to act upon the heated substance, or open at one end only when a simple sublimation is intended. Powdered substances of various kinds may be introduced into narrow tubes open at both ends, by first placing a suitable quantity of the powder in a gutter of stiff glazed paper, pushing this gutter with its contents into the tube held horizontally, then inverting the tube and gutter, and, lastly, withdrawing the gutter while still inverted. The same method sometimes be used with closed subliming tubes, so as to avoid soiling their interiors, but is unnecessary when both substance and tube are thoroughly dry. The tube, whether open or closed at one end, should be made of hard glass, and be heated in the flame of a spirit lamp or Bunsen burner. may (22.) Although quantitative analysis does not come within the scope of this work, yet a few words on weighing and measuring may not form an inappropriate addition to the foregoing remarks on chemical manipulation. The general adoption of the French metrical system, of which the gramme is the unit of weight, and the cubic centimetre or bulk of a gramme of water at its greatest density the most usual unit of measure, is highly desirable; but in default of this, the English decimal system, of which the standard grain is the unit of weight, and the bulk of a grain of water, at 62° F., the unit of measure, may be employed. In the chemical laboratory we dispense altogether with the use of ounces, drams, &c., and speak only of so many grammes and cubic centimetres, or grains and grain-measures. Mr. Griffin takes the bulk of seven grains of water as his unit of measure, which he terms a septem, so that 1000 septems are equal to one decigallon, or to the bulk of a pound of water. The use of this decimal division of the gallon is often very convenient, and quite compatible with that of the grain-measure, the septem and grainmeasure standing to one another in the simple relation of 7 to 1, as shown in the following table. The figures with a dot over One litre, or kilogramme of water-measure, equals 1.76 pints, or 61027 cubic inches. One cubic centimetre, or gramme of water-measure, equals ·06i cubic inch. Fig. 28. One decigallon or pound of water-measure equals 4535 litre, or 4535 cubic centimetres, or o'o160 cubic foot, or 27.727 cubic inches, or 16 fluid ounces. Some measures are made to contain or deliver a definite quantity of liquid. Others are graduated so that any indefinite quantity delivered may be afterwards read off. A flat-bottomed and somewhat narrow-necked flask, having a horizontal scratch across its neck marking the height to which it should be filled, forms a very convenient measure of the former kind (fig. 28). It is easy to select a couple of such flasks, which shall measure either a decigallon and half-decigallon respectively, or a litre and half-litre, &c., &c. A pipette of the form shown in fig. 29 is also a very useful instrument of this class. It is filled by carefully sucking up liquid to a level somewhat above the mark on its stem, and quickly closing its upper orifice by the finger. Then by 1 Decigallon |