CHAPTER III
Internal Seals, Air-Traps, Spray Arresters, Filter-Pumps—Sprays, Condensers; Plain, Double Surface, and Spherical—Soxhlet Tubes and Fat Extraction Apparatus—Vacuum Tubes, Electrode Work, Enclosed Thermometers, Alarm Thermometers, Recording Thermometers, "Spinning" Glass.
Internal Seals.—It is convenient to class those cases in which a glass tube passes through the wall of another tube or bulb under the heading of "Internal Seals." These are met with in barometers, spray arresters, and filter pumps, in condensers and some forms of vacuum tube. The two principal methods of making such seals will be considered first and their special application afterwards.
An Air Trap on a Barometer Tube.—This involves the use of the first method, and is perhaps the simplest example that can be given. Fig. 9, a, a1 and a2, show the stages by which this form of internal seal is made. For the first trials, it is well to work with fairly thick-walled tubing, which should be cut into two pieces, each being about eight inches long.
Fig. 9
First seal the end of one tube as described on page 13, heat the sealed end and expand to a thick walled bulb. Fuse the end of the other tube, attach a piece of glass rod to serve as a handle, and draw out; cut off the drawn-out portion: leaving an end like a.
Now heat a small spot at the end of the bulb, blow, burst out, and remove the thin fragments of glass. Heat a zone on the other tube at the point where the drawn-out portion commences and expand as shown by a1.
The next stage is to join the tubes. Heat the ragged edges of the burst-out portion until they are thoroughly rounded. At the same time heat the drawn-out tube to just below softening point. Then, while the rounded edges of the burst-out portion are still soft, insert the other tube; rotate the join in the blowpipe flame until it is quite soft, and expand by blowing. If necessary, re-heat and expand again. The finished seal, which should be slightly annealed by smoking in a sooty flame, is shown by a2.
A Spray Arrester.—This is made by the second method, in which the piece of tube which projects inside the bulb is fused in position first and the outer tube is then joined on. The various stages of making are illustrated by b, b1 and b2, Fig. 9.
A bulb is blown between two tubes by the method given on page 22, the larger tube is then cut off and the small piece of tube introduced into the bulb after having been shaped as shown in by b, Fig. 9. The opening in the bulb is sealed as shown by b1. The sealed part is now heated and the bulb inclined downwards until the inner tube comes in contact with the seal and is fused in position. This operation requires some practice in order to prevent the inner tube either falling through the soft glass or becoming unsymmetrical. The end of the bulb, where the inner tube comes in contact with it, is now perforated by heating and blowing, thus giving the form shown by b2, and the outer tube is joined on. The finished spray arrester is shown by b3. Practice alone will give the power to produce a symmetrical and stable piece of work.
Two Forms of Filter Pump.—That illustrated by d, Fig. 9, is made by the method explained under "An Air Trap on a Barometer Tube." That illustrated by c is made by the method explained under "A Spray Arrester." No new manipulation is involved, and the construction should be clear from a study of the drawings.
Multiple and Branched Internal Seals.—A fuller consideration of these will be found on page 39, but one general principle may well be borne in mind; that, as far as is possible, a tube having both ends fastened inside another tube or bulb should be curved or have a spiral or bulb at some point in its length, otherwise any expansion or contraction will put great strain on the joints.
Sprays.—A spray which is easy to make, easy to adjust, and easy to clean after use is shown by e, Fig. 9. The opening on the top of the bulb is made by melting on a bead of glass, expanding, bursting, and fusing the ragged edges. The two branches which form the spray producing junction are made by the method used for an exhaustion branch and described on page 18.
A spray which can be introduced through the neck of a bottle is shown by h, Fig. 9. The various stages in making this are illustrated by f, and g. If the inner tube is made by drawing out from a larger piece of glass so that two supporting pieces are left on each side of the place where it is intended to make the final bend, that bend can be made in a flat-flame gas burner without causing the inner tube to come in contact with the walls of the outer tube. Care must be taken when joining on the side piece that the inner tube is not heated enough to fuse it. The small hole in the side of the outer tube is produced by heating and bursting.
A Liebig's Condenser.—This consists of a straight glass tube passing through an outer cooling jacket. In practice it is better to make the jacket as a separate piece, and to effect a water-tight junction by means of two short rubber tubes. It may, however, be made with two internal seals of the class described under "A Spray Arrester." There is much less risk of these seals cracking if the inner tube is made in the form of a spiral or has a number of bulbs blown on it in order to give a certain amount of elasticity.
A Double-Surface Condenser.—Fig. 10 shows a condenser of this nature which is supplied by Messrs. Baird and Tatlock. It may be built up in stages as shown by a, b, and c, but the work involved requires considerable skill, and the majority of laboratory workers will find it cheaper to buy than to make.
Fig. 10
A Spherical Condenser.—Such a condenser as that shown by f, Fig 10, involves a method which may find application in a number of cases. The outer bulb is blown from a thick piece of tubing which has been inserted in a smaller piece (see d, Fig. 6); then the inner bulb by similar method. It is now necessary to introduce the smaller bulb into the larger, and for this purpose the larger bulb must be cut into halves. A small but deep cut is made with the file or glass-blowers' knife in the middle of the larger bulb, and at right angles to the axis of the tube on which it is blown. A minute bead of intensely heated glass is now brought in contact with the cut in order to start a crack. This crack may now be led round the bulb as described on page 30. If the work is carried out with care, it is possible to obtain the bulb in two halves as shown by d, and these two halves will correspond so exactly that when the cut edges are placed in contact they will be almost air-tight. The two tubes from the smaller bulb should be cut to such a length that they will just rest inside the larger, and the ends should be expanded. Place the inner bulb in position and fit the two halves of the outer bulb together, taking great care not to chip the edges. If the length of the tubes on the inner bulb has been adjusted properly, the inner bulb will be supported in position by their contact with the tubes on the outer bulb. Now rotate the cracked portion of the outer bulb in front of a blowpipe flame and press the halves together very gently as the glass softens. Expand slightly by blowing if necessary. If a small pin-hole develops at the joint it is sometimes possible to close this with a bead of hot glass; but if the bulb has been cut properly there should be no pin-holes formed. The condenser is finished by joining on the side tubes and sealing the inner tube through by the methods already given. In order to blow bulbs large enough to make a useful condenser, it will be convenient to employ the multiple-jet blowpipe described on page 4.
A Soxhlet-Tube or Extraction Apparatus.—This involves the construction of a re-entrant join where the syphon flows into the lower tube. It is of considerable value as an exercise and the complete apparatus is easy to make.
A large tube is sealed at the bottom and the top is lipped, as in making a test-tube. A smaller tube is then joined on by a method similar to that given on page 18, but without making a perforation in the bottom of the large tube. Heating and expanding by air pressure, first through the large tube, then through the smaller tube and then again through the large tube, will give a satisfactory finish to this part of the work.
Fig. 11
The syphon tube is now joined on to the large tube as shown by a, Fig. 11, care being taken to seal the other end of the syphon tube before joining. The details of the final and re-entrant joint of the syphon tube are shown at the lower part of a. This join is made by expanding the sealed end of the syphon tube into a small, thick-walled bulb, and the bottom of this bulb is burst out by local heating and blowing; the fragments of glass are removed and the edges made smooth by melting. A similar operation is carried out on the side of the tube to which the syphon tube is to be joined. This stage is shown by a. Now heat the syphon tube at the upper bend until it is flexible, and press the bulb at its end into the opening on the side of the other tube. Hold the glass thus until the syphon is no longer flexible. The final join is made by heating the two contacting surfaces, if necessary pressing the edges in contact with the end of a turn-pin, fusing together and expanding. The finished apparatus is shown by c.
Electrodes.—A thin platinum wire may be sealed into a capillary tube without any special precautions being necessary. The capillary tube may be drawn out from the side of a larger tube by heating a spot on the glass, touching with a glass rod and drawing the rod away; or the exhaustion branch described on page 18 may be used for the introduction of an electrode. It is convenient sometimes to carry out the exhaustion through the same tube that will afterwards serve for the electrode. The electrode wire is laid inside the branch before connecting to the exhaustion pump. When exhaustion is completed the tube is heated until the soft glass flows round the platinum and makes the seal air-tight. The branch is now cut off close to the seal on the pump side, a loop is made in the projecting end of the platinum wire, and the seal is finished by melting the cut-off end.
Platinum is usually employed for such work, but if care is taken to avoid oxidation it is not impossible to make fairly satisfactory seals with clean iron or nickel wire. Hard rods of fine graphite, such as are used in some pencils, may also be sealed into glass, but it seems probable that air would diffuse through the graphite in the course of time.
Another method for the introduction of an electrode is illustrated by d, e, f and g, Fig. 11. In this case the bulb or thin-walled tube into which the electrode is to be sealed is perforated by a quick stab with an intensely heated wire—preferably of platinum—which is then withdrawn before the glass has had time to harden, and thus a minute circular hole is made. The electrode is coated with a layer of similar glass, or of the specially made enamel which is sold for this purpose, inserted into the bulb or tube by any convenient opening, and adjusted by careful shaking until the platinum wire projects through the small hole. The bulb or tube is then fused to the coating of the electrode and the whole spot expanded slightly by blowing. The appearance of the finished seal is shown by g. It is well to anneal slightly by smoking.
Thermometers.—Apart from the notes on page 20 with respect to the blowing of a suitable bulb on capillary tubing there is little to say in connection with the glass working needed in making a plain thermometer. The size desirable for the bulb will be determined by the bore of the capillary tube, the coefficient of expansion of the liquid used for filling, and the range of temperature for which the thermometer is intended.
Filling may be carried out as follows:—Fit a small funnel to the open end of the capillary by means of a rubber tube, and pour into the funnel rather more than enough of the liquid to be used than is required to fill the bulb. Mercury or alcohol will be used in practice, most probably. Warm the bulb until a few air bubbles have escaped through the liquid and then allow to cool. This will suck a certain amount of liquid into the bulb. Now heat the bulb again, and at the same time heat the capillary tube over a second burner. The liquid will boil and sweep out the residual air, but it is necessary to heat the capillary tube as well in order to prevent condensation. Allow the bulb and tube to cool, then repeat the heating once more. By this time the bulb and tube should be free from air, and cooling should give a completely filled thermometer. Remove the funnel and heat the thermometer to a few degrees above the maximum temperature for which it is to be used; the mercury or other filling liquid will overflow from the top, and, as the temperature falls, will recede, thus allowing the end of the capillary to be drawn out. Reheat again until the liquid rises to the top of the tube, then seal by means of the blowpipe flame. The thermometer is now finished except for graduation; this is dealt with on page 75.
An Alarm Thermometer.—A thermometer which will complete an electric circuit when a certain temperature is reached may be made by sealing an electrode in the bulb and introducing a wire into the top, which in this case is not sealed. Naturally, this thermometer will be filled with mercury. There is considerable difficulty in filling such a bulb without causing it to crack.
Several elaborations of this form are made, in which electrodes are sealed through the walls of the capillary tube, thus making it possible to detect electrically the variation of temperature when it exceeds any given limits.
An Enclosed or Floating Thermometer.—The construction of this type of thermometer is shown by h and i, Fig 11. It is made in the following stages:—A bulb is blown on the drawn-out end of a thin-walled tube as shown by h. A small bulb is blown on the end of a capillary tube, burst, and turned out to form a lip which will rest in the drawn-out part of the thin-walled tube but is just too large to enter the bulb. The capillary tube is introduced and sealed in position, care being taken to expand the joint a little. The thermometer is filled and the top of the capillary tube closed by the use of a small blowpipe flame. A paper scale having the necessary graduations is inserted, and the top of the outer tube is closed as shown by i.
A Maximum and Minimum Thermometer.—If a small dumb-bell-shaped rod of glass or metal is introduced into the capillary tube of a horizontally placed, mercury-filled thermometer in such a position that the rising mercury column will come in contact with it, the rod will be pushed forward. When the mercury falls again the rod will be left behind and thus indicate the maximum temperature attained. If a similar dumb-bell-shaped rod is introduced into an alcohol-filled thermometer and pushed down until it is within the alcohol column, it will be drawn down by surface tension as the column falls; but the rising column will flow passed it without causing any displacement; thus the minimum temperature will be recorded.
Six's combined maximum and minimum thermometer is shown by b, Fig. 11. In this case both maximum and minimum records are obtained from a mercury column, although the thermometer bulb is filled with alcohol. It is an advantage to make the dumb-bell-shaped rods of iron, as the thermometer can then be reset by the use of a small magnet, another advantage consequent on the use of metal being that the rods can be easily adjusted, by slight bending, so as to remain stationary in the tubes when the thermometer is hanging vertically, and yet to move with sufficient freedom to yield to the pressure of the recording column.
The thermometer may be filled by the following method:—When the straight tube has been made the first dumb-bell is introduced and shaken down well towards the lower bulb, the tube is now bent to its final shape and the whole thermometer filled with alcohol as described on page 44. Now heat the thermometer to a little above the maximum temperature that it is intended to record, and pour clean mercury into the open bulb while holding the thermometer vertically. Allow to cool, and the mercury will be sucked down. The second dumb-bell is now introduced, sufficient alcohol being allowed to remain in the open bulb to about half fill it, and the alcohol in this bulb is boiled to expel air. The tube through which the bulb was filled in now sealed.
Clinical Thermometers.—The clinical thermometer is a maximum thermometer of a different type. In this case there is a constriction of the bore at a point just above the bulb. When the mercury in the bulb commences to contract, the mercury column breaks at the constriction and remains stationary in the tube, thus showing the maximum temperature to which it has risen.
Vacuum Tubes.—There are so many forms of these that it is scarcely practicable or desirable to give detailed instructions for making them; but an application of the various methods of glass-working which have already been explained should enable the student to construct most of the simpler varieties. An interesting vacuum tube is made which has no electrodes, but contains a quantity of mercury. When the tube is rocked so as to cause friction between the mercury and the glass sufficient charge is produced to cause the tube to glow.
A Sprengel Pump.—This, in its simplest form, is illustrated by a, Fig. 12. Such a form, although highly satisfactory in action, needs constant watching while in action, as should the mercury funnel become empty air will enter the exhausted vessel. Obviously, the fall-tube must be made not less than thirty inches long; the measurement being taken from the junction of the exhaustion branch with the fall-tube to the top of the turned-up end.
Fig. 12
A Macleod Pump.—One form of this is illustrated by b, Fig. 12. It has the advantage that the mercury reservoir may be allowed to become empty without affecting the vacuum in the vessel being exhausted.
"Spinning" Glass.—By the use of suitable appliances, it is quite possible to draw out a continuous thread of glass, which is so thin as to have almost the flexibility and apparent softness of woollen fibre; a mass of such threads constitutes the "glass wool" of commerce.
The appliances necessary are:—a blowpipe capable of giving a well-formed flame of about six or eight inches in length, a wheel of from eighteen inches to three feet in diameter and having a flat rim of about three inches wide, and a device for rotating the wheel at a speed of about three hundred revolutions per minute.
A very satisfactory arrangement may be made from an old bicycle; the back wheel having the tyre removed and a flat rim of tin fastened on in its place. The chain drive should be retained, but one of the cranks removed and a handle substituted for the remaining pedal. The whole device is shown by Fig. 13.
Fig. 13
The procedure in "spinning" glass is as follows:—First melt the end of a glass rod and obtain a large mass of thoroughly softened glass, now spin the wheel at such a speed that its own momentum will keep it spinning for several seconds. Touch the end of the melted rod with another piece of glass and, without withdrawing the original rod from the blowpipe flame, draw out a thread of molten glass and twist it round the spinning wheel. If this is done properly, the thread of glass will grip on the flat rim, and by continuing to turn the wheel by hand it is possible to draw out a continuous thread from the melted rod, which must be advanced in the blowpipe flame as it is drawn on the wheel. If the rod is not advanced sufficiently the thread will melt off, if it is advanced too much, so as to heat the thick part and allow the glass to become too cool at the point of drawing out, then the thread will become too thick, but it is easy after a little practice to obtain the right conditions. Practice is necessary also in order to find the right speed for the wheel.
When sufficient glass has been "spun," the whole "hank" of thin thread may be removed by drawing the thumb-nail across the wheel at any point on its flat rim, thus breaking the threads, and allowing the "hank" to open.
Brushes for Use with Strong Acids.—Glass wool, if of fine enough texture to be highly flexible, can be used to make acid-resisting brushes. A convenient method for mounting the spun glass is to melt the ends of the threads together into a bead, and then to fuse the bead on to a rod; thus giving a brush. If a pointed brush is necessary, the point may be ground on an ordinary grindstone or carborundum wheel by pressing the loose end of the spun glass against the grinding wheel with a thin piece of cardboard.
When using brushes of this description, it is well to bear in mind the fact that there is always a liability of a few threads of glass breaking off during use.