CHAPTER IV
Advanced Exercises
EXERCISE NO. 8
Sealing a Tube Through Another Tube
First Method—Making a Gas-washing Tube
This first method can be used whenever one can work through an open end opposite to the end of the tube where the joint is to be made. To illustrate it, take a piece of rather thin-walled tubing, about 3⁄4 inch in diameter, and some pieces of rather strong tubing a little less than 1⁄4 inch in diameter. Draw off the large tube in a short cone, then draw off the tail as in the making of the bulb on the end of the tube, blow out the little lump slightly, shrink the whole cone a little and blow gently to form a rounded end like that on a test-tube, with walls about the thickness of those of the rest of the tube. Cut this tube to a suitable length, say about six inches, and provide two corks which will fit the open end of it. Now cut a piece of the small tubing of the proper length to form the piece which is to be inside the large tube. For practice purposes, this piece should be about an inch shorter than the large tube. Flange one end of this tube a little, and anneal the flange well in the smoky flame. Bore one of the corks so that a piece of the small tubing will fit it, and cut a couple of notches in the side of this cork so that air can pass between it and the glass. Pass a short piece of the small tubing through this cork, and attach the flanged piece of small tube to this by means of a short piece of rubber tubing, so that when the whole is inserted in the large tube it is arranged as in a, Fig. 11. The piece of glass tubing projecting out through the cork is now cut off so as to leave an end about 1⁄2 inch long when the cork is firmly seated and the inner tube pushed into contact with the center of the end of the large tube, as shown in the drawing. Care should be taken that the little rubber tube which joins the two pieces is arranged as in the figure; i.e., most of it on the piece of tubing which passes through the cork, and very little on the other piece, so that when the cork is removed after the small tube has been sealed through the large one, the rubber tube may easily come with it. Select a short piece of the small tubing of suitable length for the piece which is to be on the outside of the large tube as a continuation of the piece inside, and another piece for the delivery tube. A small bulb may be blown in the latter at a point about 2-1⁄2 inches from the closed end, and the open end cut off about 1-1⁄2 inches from the bulb. A cork or cork-boring of suitable size to stopper the small tube is prepared, and laid ready with the other (unbored) cork for the large tube.
Fig. 11.—Gas-washing tube.
When everything is in readiness, the rounded end of the large tube is slowly heated until it softens and joins firmly to the small tube inside. After it has shrunk down well, it is blown out to its original size, placing the whole end of the large tube, cork and all, in the mouth. Now with a fine-pointed flame the glass covering the end of the small tube is heated to the softening temperature, and then is blown out to an excrescence by blowing on the end of the small tube which passes through the cork. The end of this excrescence is heated and blown off in the usual way, so as to leave the small tube sealed on the inside of the large one and opening through it into this short tube which has been blown out. The end of the small tube which passes through the cork is now closed with the cork prepared for it, and the short outer tube is joined to the tube that has just been blown out, so that the joint appears like b, Fig. 11. Use the first method (Exercise No. 1) for this joint. Reheat the whole of the end of the tube nearly to the softening temperature, anneal it a little, and allow to cool a few seconds until well set. Now remove the cork, short glass tube and rubber tube from the open end of the large tube and insert the solid cork in their place. Warm the joint and the whole of that end of the tube again carefully up to about the softening point, then seal on the side tube for the delivery of the gas in the usual way, taking care that the whole of the end and the joint are kept warm meanwhile. When thoroughly sealed, the delivery tube is bent up parallel to the tube through which the gas enters, and then out at right angles to it, as shown in c. The whole of the end of the tube is now cautiously reheated and then cooled slowly to anneal it.
The cork may now be removed from the open end of the large tube, this end heated in a large flame, caught together with a scrap of glass tubing and drawn off into a cone so that the base of the cone is about opposite the end of the inner tube. The lump of glass is drawn off the point of this cone and it is reblown to form a rounded end, as previously described.
After this cools, the tube through which the gas enters may be heated at the proper point and bent at right angles to form the finished apparatus as shown in d. The ends of the small tube are cut off square and fire-polished.
Discussion.—After the joint has once been made, great care must be taken that it is kept hot during all the subsequent manipulations, and if it becomes somewhat cool at any time it must be reheated very slowly. It is obvious that the rate of heating and cooling of the inner tube will be slower than that of the outer tube, and this will readily produce stresses which tend to crack the tube at the joint. The amount of heating and cooling which such a joint will stand depends upon its form. The beginner should examine such a joint on regular factory-made apparatus, and note the uniformity of wall-thickness and the "clean-cut" appearance of the joint, as a model for his imitation. A ragged joint, where the line of joining of the inner and outer tubes wavers instead of going squarely around the tube, is almost sure to crack during the cooling and heating unless extra precautions are taken with it. The presence of a small lump of glass at any point on the joint affords an excellent starting place for a crack, as do also the points on a ragged joint where the inner tube comes farther down on the outer tube than at other points.
In order to insure a joint which is square and not ragged, it is essential that the angle between the inner and outer tubes at the joint be very nearly a right angle. For this reason the two tubes should not be of too near the same size, or if this cannot be avoided, a small bulb should be blown on the end where the joint is to be made. If this bulb be made with the same wall-thickness as the rest of the tube, and somewhat pear-shaped, it may be drawn out to the same size as the rest of the tube, if necessary, after the joint has been made.
This method is used wherever possible in preference to the second method (Exercise No. 9), as it is easier to get a good joint with it. It may also be used where it is desired to seal the tube through the side of a tube, or for a tube sealed through the wall of a bulb, as in a Geissler potash bulb or similar apparatus. Where there is not space to join the inner tube to the blowing tube by a rubber tube, this joint may be made with a small piece of gummed paper, which can readily be broken when desired.
EXERCISE NO. 9
Sealing a Tube Through Another Tube
Second Method—Making a Suction Pump
Select a piece of tubing 3⁄8 to 1⁄2 inch in diameter, with walls about 1⁄16 inch or a little less in thickness, heat a place about 4 inches from one end and draw it out so that when cut off at the proper point it will look like a, Fig. 12; the open end of the drawn out part being small enough to slip inside another piece of the original tube. A small thick-walled bulb is now blown as indicated by the dotted lines, and annealed. A piece of the original tubing is now prepared, 7 or 8 inches long, with one end cut square off and the other closed. A piece of 1⁄4-inch tubing about 2 inches long, and drawn out at one end to a tail several inches long is also prepared, to form the inlet tube for the air. Another piece of the 3⁄8-inch tube is prepared, about 4 inches long, and provided with a tail drawn out as indicated in b, so that when cut off at about 2-1⁄2 or 3 inches from the main tube its inner diameter may be slightly less than that of the narrowest point of the tube a. A small thick-walled bulb is blown at the point indicated by the dotted lines, and annealed. Care must be taken in drawing the capillary and blowing the bulb in both a and b that the capillary tubes are in the axis of the main tube, and in the same straight line with it.
Fig. 12.—Suction pump.
The open end of the 8-inch piece of tube and the bulb of the piece a are now warmed together, the end of the tube only moderately and the bulb to about its softening temperature. The tube a is now inserted in the open end of the large tube, and the bulb softened with a suitable flame and pressed into good contact with the tube. It is then reheated, including the joint, blown a little and pulled out to form a straight tube in line with the main tube. By warming the joint a little, and proper rotation, the capillary may be brought into the same straight line with the rest of the tube.
Keeping this joint hot, a place about an inch from it on the tube a is warmed, and the piece of 1⁄4-inch tubing previously prepared is sealed on at that point. The joint is then well annealed and allowed to cool.
The tube a is now cut at such a place that when b is inserted in the open end the point will come near the end of the constriction of a, as shown in c. Care is taken to get a clean square cut. The side tube is now cut off about an inch from the main tube and corked. Tube b is sealed into the open end of a, in the same way as a was sealed into the large tube, and the joint carefully annealed.
Discussion.—As in the first method, the secret of success lies in getting a square joint, and having the inner tube leave the outer one at nearly right angles. All the remarks about annealing, lumps, etc., made under the previous method apply here.
This method may be applied in sealing a small tube into the end of a large one, the latter being either drawn to a cone and cut off at the desired diameter, or else given a rounded end like a test-tube and a hole the proper size blown in the center of it. A suitable thick-walled bulb is to be blown on the small tube, as in the case described above. This method is also used in making the Kjeldahl trap (a, Fig. 13), the small tube to be inserted being first drawn, the thick bulb blown at its point of union with the main tube, and then the small tube bent and cut. The large bulb is best made with rather heavy wall, being either blown in the middle of a tube, and one piece of the tube drawn or cut off, or else made on the end of a tube. In the latter case a drop of glass must be put on the point where the joint is to be, so as to get a hole of the proper size with enough glass around it to prevent it from growing larger when it is heated. The author prefers to blow the bulb in the middle of the tube, draw off one end of the bulb, and blow out the desired hole where the tube was drawn off. The whole bulb must generally be reheated and blown a little at the end of the process, and well annealed.
Fig. 13.—a, Kjeldahl trap; b, suction pump on smaller tubing.
The suction pump can also be made on 1⁄4-inch tubing, and one joint saved if desired, by constricting the tube to form the raceway for the water and air, as shown in b, Fig. 13. (See page 10 for method.) But it is more difficult to make a square joint on such small tubing.