When well shrunk and heated to blowing temperature the joint is removed from the flame and blown gently with careful rotation, pushing the tubes together a little when the blowing is about finished, so that the cone becomes a short thick half-bulb, as shown in d, Fig. 8. This corresponds to the first thick bulb in the first method (d, Fig. 6), and is treated similarly. It is again heated and shrunk, taking care not to involve either the large tube or the small one in the shrinking, blown quickly to about the same shape as before, (d, Fig. 8), and then gently drawn out into a smooth cone (e), exactly as in the first exercise. Care should be taken not to draw too rapidly or too far, as then the resulting cone (f) is weaker than it should be, and does not look well.
Discussion.—The beginner will find that this operation is best learned on two tubes which are not too nearly of the same diameter. A tube about 5⁄8 inch in diameter and one a little less than 1⁄4 inch will be suitable. Both should have moderately heavy walls (1⁄16 inch or a trifle over for the large tube, and a trifle less for the small one) but the large tube should not be too heavy or else it will be hard to prevent melting down too much of the small tube, and getting this drawn out too thin during the process. One of the troublesome features of this exercise is the difficulty of rotating two tubes of different diameters with the same angular velocity, so as not to twist the joint. Another difficulty is found in getting the cone uniformly heated to blowing temperature without overheating and overshrinking the small tube. The reason for this is obviously the much greater circumference of the cone, especially at its large end, so that relatively much less of it is being heated at any time. The beginner is also inclined to start with too long a cone, or else heat so much of the large tube that part of its glass is included in the cone, with the result that in order to get the right wall-thickness the cone must be made too long (g, Fig. 8). This does not look well, and usually will be irregular in shape.
EXERCISE NO. 5
Tube for Condensing Sulphur Dioxide
This is useful as a test of mastery of the preceding exercise. A piece of 3⁄16 or 7⁄32 inch tubing is joined to each end of a piece of tubing 5⁄8 by about 5 inches, and two constrictions made in the large tube, by the method described on page 10. The small tubes are then bent in the same plane, as shown, and their ends fire-polished (Fig. 9).
Fig. 9.—Tube for condensing sulphur dioxide.
EXERCISE NO. 6
Bulb at the End of a Tube
For this exercise tubing of 1⁄4 inch diameter and moderately strong walls is selected. A tail is drawn out on one end of the tube, and a piece of tubing about nine or ten inches long is cut off. The tail should be carefully drawn in the axis of the tube, and in the same straight line with it, as it is to be used as a handle in assembling the glass for the bulb. This tail must be long enough so that it can be conveniently held in the left hand, as described on page 13, and rotated about the same axis as the main tube. Holding the main tube in the right hand and the tail in the left, the tube is rotated in a large flame so that a piece of it, beginning where the tail stops and extending about an inch to the right, may be uniformly heated to the highest temperature at which it can be kept in shape. As soon as this temperature is reached, the tube is removed from the flame, continuing the rotation and taking care not to draw out the heated part, and gently blown. The rotation is carefully continued during the blowing, holding the tube in approximately a horizontal position. As soon as the tube has expanded a little the tail is pushed gently toward the main tube, continuing the gentle blowing. If this is properly done, the heated piece of tube will become a short bulb of about double its original diameter, and about the same wall thickness as the original tube. It will have somewhat the appearance of a, Fig. 10, when properly manipulated.