Sealing on a New Tube.—It frequently happens that one of the tubes of the stopcock is broken off close to the cock itself, and a new one must be joined to the stub of the old one. With care, this may often be successfully done even where the break is within 1⁄4 inch of the stopcock. The first step is to clean and dry the stopcock, remove the plug, cork the open ends of the stopcock sleeve and the other tube, and wind a couple of layers of asbestos cord carefully over the sleeve and the most of the corks which close it. A suitable tube, having as near as possible the same diameter and wall strength as the one broken off, is selected and a piece the desired length cut off. The broken end of the tube on the stopcock is now squared off as well as possible, by cutting or by heating and drawing off the projections, and the new tube sealed on, usually with the first method (Exercise No. 1). If the break is very close to the stopcock, very little reheating and blowing can be done, on account of the danger of getting the stopcock sleeve out of shape, and the work must be heated very slowly to prevent cracking. The main reliance is then placed on making a good joint when the tubes are brought together, and then drawing out this joint a little, at once, to get an even wall.
CLOSED CIRCUITS OF TUBING.
In some pieces of apparatus closed circuits of circular or rectangular shape are required. A similar problem is involved in apparatus like the ordinary Soxhlet extractor, where a small tube is joined to the side of a large one, bent to form a siphon, and attached again to a continuation of the original large tube. The difficulty in all such cases is to provide for the contraction taking place as the last joint cools. If part of the circuit has the shape of the letter S, or is a spiral, the natural springiness of the glass will take care of this. If not, the side of the circuit opposite to the joint and parallel to it must be heated also, the two being finally heated together to the softening point after the joint is completed, and then allowed to cool together.
To make the last joint, the rest of the tube is made in approximately the desired form, the two pieces which are to be joined to make the last joint being just enough out of the desired position to allow them to pass one another. The final joint is preferably made in the middle of a straight piece of tube, not at a tee. The two pieces which are to be joined are bent so as to just pass each other, marked at the right point with the glass-knife, and cut there, preferably with a small bead of hot glass. One or both of these tubes are now warmed to the softening point in such a place that the tubes can be made to meet properly, and the two cut ends pressed together. They are now warmed in the flame, and joined together, either by simultaneously warming the opposite side of the circuit or some other suitable part, so as to allow the two ends to be pushed together again after they are softened, or by gently touching the places that do not unite with a hot bead of glass, and using the glass to fill up the crack where the ends do not quite meet. Care must be taken not to leave knots or lumps of glass in the finished joint, and the latter should be well reblown, and if necessary left as a small bulb or enlargement, rather than have it have too thick walls.
SPIRALS
Spirals of glass tubing are probably best made free-hand before the blow-pipe, unless one has a great many of them to make, and extreme accuracy is desired. To begin with, a piece of tubing of the desired size (say 3⁄16 inch in diameter) and a convenient length (about two feet) is selected, one end closed, and a right-angle bend made about six inches from the closed end. Holding the closed end in the left hand and the long open one in the right, the spiral is begun. The short closed end is to be parallel to the axis of the spiral, and preferably in that axis. Using a moderate-sized flame, of somewhat yellow color, and taking care to heat the whole circumference of the tube, the long open end is wound little by little into a spiral having the short end a (Fig. 16) as an axis. The bend at b, where the tube changes from the radius to the circumference of the circle, must be rather short, but the tube must not be flattened or constricted here. Especial pains is to be taken with the first turn of the spiral (b to c, Fig. 16), as the shape of this determines the diameter of the whole spiral, and serves as a guide for the rest of the turns. The winding of the tube is best accomplished, after a portion has been softened, by slowly turning the short end a a little about its own axis, while the long open end remains where it was. This winds the tube into a spiral, just as if there were a solid cylinder in the center of it, and this cylinder was being turned about its axis, and was winding up the soft glass upon its circumference. As the cylinder is not actually there, the curve of the turns must be carefully estimated by the eye, so that the spiral may be uniform and moderately smooth. When the original piece of tube has been used up, another piece is sealed on to the open end, and the operation continued as far as may be required.
Fig. 16.—Making a spiral.