ratus, the tube has to be bent at right angles; but for syringe-bottles, small tubeapparatus, &c., it requires bending at more or less acute angles; the mode of manipulating is, however, in all cases the same.

For ordinary, and even for tolerably hard glass tubing, provided it be not very large or thick, a simple spirit-lamp, or bat's-wing gas-jet, may be used as the source of heat. The tube is supported on each side of the portion to be heated, and held over the flame for a short time, being turned round continually, in order that it may become uniformly heated, and also moved slightly backwards and forwards so as to heat a sufficient surface. When the tube has become thoroughly heated, it is introduced into the upper portion of the flame, and continually moved about as before until it is soft to the touch, and has been uniformly heated to the length of about an inch, or even more, if a very round bend is required; by a gradual pressure, the requisite inclination is then given. The principal precautions to be observed in bending tubes, are: not to have the

Fig. 36.

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Fig. 37.

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glass too soft, and to heat it uniformly, or the bend may become distorted and flattened at the convex portion (Fig. 36); to give no other than a downward direction to the hand, so that the two extremities of the tube remain in corresponding directions; to heat a sufficient length of the tube, so that the bend may not be at a sharp angle, but round (Fig. 37); to apply only a very slight pressure in bending, otherwise, as soon as the tube becomes too cool to yield, it will snap. If a very round bend is required, one portion of the glass must be bent first as far as admissible, without affecting the bore of the tube or distorting it; the neighboring portions are then heated, and the bend continued in the same manner until the requisite curve is attained. Very thin tubes must always be bent very round, or else the convex portion of the bend is certain to become flattened, and so thin as to be very fragile when cool, and the concave portion will bend in folds, and become otherwise distorted. If gas is at hand, the ordinary fish-tail burner affords a most convenient flame for making round bends, as a large surface of tube may be heated by it at one time. Unless the tube has been heated too powerfully, no difficulty will be found in removing, when cool, any carbon that may have been deposited upon it by the gas. Should the tube to be bent be of larger dimensions, and consist of glass not easily fusible, the table or the Herapath mouth-blowpipe must be employed for heating it, the same precautions being observed as above directed.1

Before introducing tubes into the corks, it is necessary to round the sharp edges of the extremities of the former, which would otherwise cut and injure the

Should the bend required be so near one extremity of the tube as to preclude the possibility of supporting the short end with the fingers, or should it be even at the very extremity, the force necessary on this side must be applied by pressing against the glass with an iron rod; the latter should, however, not be applied until the moment that the curvature is to be made, nor retained in contact with the glass so long as to reduce its temperature below the point of softness, as there would be great risk of cracking the glass under those circumstances.

perforations prepared for their reception. This is effected by fusing the extremities, either in the flame of an ordinary lamp or by the blowpipe. This operation requires considerable care, on account of the great facility with which tubes crack at the extremities. It is necessary first to heat the tube gradually, as directed above, to the extent of about an inch from the termination, and to remove it slowly from the flame until the extreme end only remains therein. As soon as the edge becomes faintly redhot, the tube should be entirely removed, as otherwise it will contract at the extremity, which is sometimes very inconvenient. After having heated a tube, the operator must be careful to avoid placing the heated portion upon any cold surface, such as the table, until it has cooled down considerably. The force applied in pushing the tube into the cork must be very gradual and gentle; the best plan is to screw it in; it is also advisable, when the tubes fit rather tightly in the perforations, or when the former are rather large, to grease them slightly at the extremity first introduced.

When the tubes are properly inserted, the cork is fitted into the vessel with great care; it being held firmly between the thumb and two first fingers, and screwed round, a gradual downward pressure being simultaneously applied.

The cork having been thus pressed into the vessel as far as possible, it is requisite to ascertain whether the fitting is air-tight. Should the cork contain more than one tube, it is necessary, in order to do so, to close the exterior openings of all but one. The mouth is then applied to the open tube, a portion of the air sucked out of the vessel, and the tongue immediately placed against the opening. If the apparatus is tight, the tongue will be forced into the aperture. Or, instead of sucking out the air, an extra portion may be forced into the vessel from the mouth, and the tongue then pressed against the opening to prevent its escaping from that quarter. If there be any leakage in the vessel, the air will be heard to issue from the place at which the leakage exists, and the pressure against the tongue will gradually diminish. By the latter test, the precise spot at which the apparatus is defective may be easily ascertained.

It is always preferable, if possible, to fit up an apparatus perfectly air-tight with good corks alone; should this, however, not be practicable (which is frequently the case when the corks are large, or the openings of the vessels not perfectly round), recourse must be had to luting.

The

§ 26. LUTES.-Various substances may be employed as luting to corks. most convenient are, almond-paste, linseed-meal (or a mixture of both), whitelead, and plaster of Paris. If the leakage of a cork be but slight, and in the substance of the cork itself, it may be stopped by the application of a solution of sealingwax in spirits of wine, which penetrates into and fills up the small pores. But if the leakage be found to exist at any point where the cork touches the glass, it is advisable to cover the whole cork with one of the lutes above mentioned. The compactness and adhesiveness of linseed and almond lute is much increased by the addition of a little alkali to the water with which the meal is mixed. It is also advantageous to dissolve a small portion of glue in the water employed in making plaster of Paris luting.

If the gas to be generated may be at once collected, without undergoing any purification, the apparatus generally consists of a flask containing, tightly fitted, by means of a cork, a funnel or safety tube, and a delivery-tube, which last is either bent twice downwards (the short end, which fits into the cork, at a right angle, and the long end at a more or less obtuse angle); or it is bent once, in the form of the siphon, the bend being a very round one, and the short arm fixed into the cork.

If the gas is to be collected over a pneumatic trough, the long end of the tube is bent upwards at the extremity, at an angle of about 65°. The same form of tube may be used if the gas is to be collected in gas-holders; but it will be

found far more convenient to shorten the delivery-tube, and attach it to a piece of vulcanized Indian-rubber tubing of sufficient length.

Very frequently, the gases require purification previously to collection or use; in that case, the long arm of the delivery-tube is likewise bent at right angles, and connected with such purifying apparatus as will be presently described.

If only a small quantity of gas requiring heat for its disengagement is to be prepared, much saving of time and material is effected by the use of a test-tube, or very small flask, in the place of the larger flask, into which is fitted, by means of a perforated cork, the short arm of a tube bent in the form of a siphon, the extremity of the long arm being bent upwards, as described above. This little apparatus may be conveniently held over a spirit-lamp by means of a test-tube holder, and requires no safety-tube, as it may be removed from the water into which the delivery-pipe dips, the moment the evolution of gas ceases.

APPARATUS FOR THE GENERATION OF GASES AT ORDINARY TEMPERATURES.

§ 27. When no application of heat is necessary to aid the evolution of a gas, two different forms of vessels may be employed as generators, namely, the Woulfe's bottle, and corked wide-mouthed bottles.

THE WOULFE'S BOTTLE.-This very convenient piece of apparatus consists of a bottle with two or three separate openings, which may be fitted with corks of the ordinary size. It presents a great advantage over common glass bottles, as two or three tubes may be fitted into it perfectly air-tight, with great ease. When required for generating gas, it need only have two openings, one for the funnel or safety-tube, and the other for the delivery-tube. Great care should be taken that the interior of the necks of these bottles be perfectly cylindrical. When Woulfe's bottles cannot be obtained, wide-mouthed bottles may be substituted, provided they can be fitted with good bungs, to which tubes may be adapted in the manner before directed. The principal difficulty attending their use, is that of fitting them air-tight, since large corks can be but rarely obtained free from flaws, or sufficiently elastic to allow of proper softening. It is therefore almost always necessary in their use to have recourse to luting.

ARRANGEMENTS FOR PURIFYING GASES.

28. It is frequently necessary to subject the gases to purification previously to collecting them, in order to remove trifling admixtures of other gases, or particles of liquids which are frequently held in mechanical suspension. This is effected, either by allowing the gas to pass through liquids of various descriptions, or over solids; the former being contained in common or in Woulfe's bottles, properly fitted up with connecting-tubes, &c.; the latter, in tubes of

various forms.

If a Woulfe's bottle with three necks is employed, the centre neck is fitted with a safety-tube, passing nearly to the bottom of the vessel; into the neck on the one side is fitted a tube bent at right angles, and reaching likewise nearly to the bottom of the vessel, while the third neck also contains a tube bent at right angles, but protruding into the vessel only about half an inch beyond the cork. With the latter tube is connected (in a manner to be presently described) either another purifying vessel, or the delivery-tube. Should the Woulfe's bottle only have two necks, it is advisable to fit into one a piece of tube sufficiently wide to admit conveniently the delivery-tube of the generator. This wide tube should reach to within about half an inch of the bottom, and be cut off obliquely, or slightly notched at the lower extremity. This not only acts as a safety-tube, but also presents a movable joint, which is very convenient, since the gene

rating apparatus may be turned about, or detached from the remainder, with great ease.

In the absence of Woulfe's bottles, such wide-mouthed bottles as have been described already may be fitted up in a similar manner, and answer the purpose very well, provided the fittings are perfectly tight.

CONNECTION BY MEANS OF CAOUTCHOUC JOINTS.-The bottles just described, having been properly charged with the purifying agent (with which they should not be more than half filled), are connected with the generating apparatus (and with each other, if more than one be required) by means of caoutchouc connecting pieces. These little tubes, which are indispensable to every gas-apparatus, since they impart to it the necessary flexibility to permit of the individual portions being moved about and disconnected from the remainder with ease, are very easily made of sheet caoutchouc, about the tenth or twelfth of an inch thick, in the following manner: A piece of this caoutchouc of the required length is gently warmed, so as to render it perfectly flexible and soft; it is then put round a piece of glass tubing or rod, not quite the size of the intended connector. The portions that project on either side are pressed together so as to adhere pretty closely; they are then cut off with a very sharp pair of scissors. The two edges are thus obtained perfectly clean, and adhering slightly to each other; they are now pressed closely together, care being taken not to soil the cut edges. When this operation is

Fig. 40.

neatly and properly performed, the two fresh surfaces join accurately together, and provided they are perfectly clean, they will adhere to each other so firmly, that the tube will tear quite as easily at any other place as at the junction. It will frequently happen that the connector will adhere so firmly to the glass on which it is made, as to render it extremely difficult to remove it without fracture. The application of a small quantity of flour, or other fine powder, to the inner surface of the caoutchouc will prevent this; or should this have been neglected, a drop of water, held to the one extremity of the tube, will be immediately sucked in between it and the glass, when it may then be removed with great Should there be any slight defect in the connector, it is well to make a

ease.

second one upon it, the joint of which should be made on the opposite side to that of the inner one; the two tubes may be made to join at the extremities, by clipping a small quantity off; this must not be done at one cut, as the tube would then be closed at the ends, but the scissors should move round the tube.

Small pieces of vulcanized Indian rubber tubing, which is now made of almost any dimensions, answer the purpose of these connectors exceedingly well; they may not adhere to the glass quite so tightly, a defect which may, however, be remedied, by tying them firmly upon the tubes at each extremity.

In connecting the various portions of a gas-apparatus by means of these joints, the ends of the glass tubes should be about the sixth or eighth of an inch distant from each other, inside the connector, in order to impart to the apparatus a proper degree of flexibility.

The tying of these joints upon the glass tubes, should they need it, requires some care. The best material for tying is silken cord, of moderate thickness. The force employed in tightened and tying the cord round the connector should be very moderate; as, otherwise, the safety of the inclosed glass tube is endangered.

It is sometimes necessary to pass gases over solids of different descriptions, in the state of lumps or powder. These solids are contained in straight tubes, or in tubes bent in the form of the letter U. The straight tubes are employed of various diameters (from to inch) and lengths, according to the substance they are to contain, and are fitted at each extremity with a good cork, containing a piece of narrow glass tubing about two inches in length, and protruding about one-eighth of an inch through the cork; these serve to connect them, with the aid of caoutchouc joints, to any form of apparatus. It is advisable to place against the inner extremity of each narrow tube a small piece of cotton-wool or asbestos, in order to prevent any particles of the solid in the tube from being carried out by the current of gas. If the solid is in powder, the tube should not be filled too full, in order that a small passage may be formed between the upper part of the latter and the powder, by knocking the tube lengthwise upon

the table.

Should the solid used be in the state of lumps, they should be of moderate size, smaller fragments being introduced alternately with larger, in order that no large space may be left in the tube.

If it is necessary to weigh these tubes, it is better to cut the corks off even with the tube, and to coat them with sealingwax, to prevent their absorbing moisture, and thus altering the weight of the apparatus.

The U-tubes are only employed for solids in the state of lumps, and are filled and fitted up in the same manner as the straight tubes, excepting that the narrow tubes which are fitted into the corks are not straight, but bent at right angles.

APPARATUS FOR COLLECTING GASES.

§ 29. Gases are, with few exceptions, collected over water, unless the dry gas be required, when mercury is employed in its stead.

When a gas is prepared in any considerable quantity, or it is wished to preserve it for any length of time, it is collected in gasometers, or gas-holders.

The former have been almost entirely superseded in the laboratory by the Pepys's gas-holder; we shall, therefore, confine ourselves to a brief description of the latter.

THE GAS-HOLDER.-This apparatus consists of a closed cylindrical vessel, usually of copper, which is surmounted by a circular, shallow trough of the same diameter, resting upon four pillars. Into the centre of the bottom of the trough is fixed a pipe, which passes through the top of the gas-vessel, and reaches nearly to the bottom.