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.