When a bulb has to be blown upon a very fine tube, for example upon thermometer tubing, the mouth should not be employed, for the moisture introduced by the breath is extremely difficult to remove afterwards. A small india-rubber bottle or reservoir, such as those which are used in spray-producers, Galton’s whistles, etc., securely attached to the open end of the tube, should be used. With the help of these bottles bulbs can be blown at the closed ends of fine tubes with ease, though some care is necessary to produce them of good shape, as it is difficult to rotate the hot glass properly when working in this way.
Making and Grinding Stoppers.—Apparatus which is to contain chemicals that are likely to be affected by the free admission of air, needs to have stoppers fitted to it. Making a good stopper is a much less tedious process than is commonly supposed.
Fig. 23.
Suppose that the tube I. of [Fig. 23] is to be stoppered at A, it must be slightly enlarged by softening the end and opening it with a pointed cone of charcoal; or a conical mouth for the stopper may be made by slightly contracting the tube near one end, as at B, cutting off the cylindrical end of the tube at the dotted line C, and then very slightly expanding the end at C with a charcoal cone after its edges have been softened by heat. In either case the conical mouth should be as long and regular as possible.
For the stopper take a piece of rather thick tube, of such size that it will pass easily, but not too easily, into A or B. Expand this tube at D, as shown in II., by softening the glass and gently compressing it. The configuration of the enlarged tube as shown at D may be obtained by heating and compressing two or more zones of the tube that are adjacent, one zone being less expanded than the other, so as to give the sides of the imperfect stopper as nearly as possible the form shown at D, which, however, is much less regular than may easily be obtained. Seal off the head of the tube at H, and heat the glass till it runs together into a nearly solid mass; compress this with a pair of iron tongs to the flattened head E. In making D, aim at giving it a form which will as nearly as possible correspond to that of the tube into which it is to be ground, and make it slightly too large, so that only the lower part at D can be introduced into the mouth of A or B. Before it is ground, the stopper must be heated nearly to its softening-point and annealed.
Moisten D with a solution of camphor in recently distilled turpentine, and dust the wet surface with finely-ground emery, then gently grind it into its place till it fits properly. In this operation the tail G, which should fit loosely into the tube A, will be of assistance by preventing D from unduly pressing in any direction on A in consequence of irregular movements. The stopper should be completely rotated in grinding it. It must not be worked backwards and forwards, or a well-fitting stopper will not be produced. Renew the emery and camphorated turpentine frequently during the earlier part of the grinding; when the stopper almost fits, avoid using fresh emery, but continue to remove the stopper frequently at all stages of the operation. That added at the earlier stages will be reduced to a state of very fine division, and will therefore leave the stopper and mouth of A with smoother surfaces than fresh emery.[10]
Note.—The addition of camphor to the turpentine used for grinding glass is very important. Notwithstanding its brittle nature, glass will work under a file moistened with this solution almost as well as the metals. Small quantities should be made at a time, and the solution should be kept in a well-closed vessel, for after long exposure to the air it is not equally valuable.
If the stopper is to fit a tube contracted like B, it must be constructed from a piece of tube that will pass through the contraction at B. The tail GF will not do such good service as it does in the case of a tube which has been opened out to receive its stopper, but it will help to guide the stopper, and should be retained.