These joints are very apt to break after a few minutes or hours if the glass of D be much thicker than that of the bulb A. They should be wrapped in cotton wool for annealing as soon as possible, as the rate at which the tube E cools is likely to be less rapid than that of the parts of the apparatus which are more freely exposed to the air; therefore all such internal joints require very careful annealing, and they should always be made as thin as is consistent with the use to which they are to be put.
Tubes may also be sealed into the ends or sides of larger tubes by piercing them at the point at which the inserted tube is to be introduced, and proceeding as in the case of the air-trap just described.
Ozone generators of the form shown on next page ([Fig. 19]), afford an interesting example of the insertion of smaller tubes into larger.
On account of the small space that may be left between the inner and outer tubes of an ozone generator, and of the length of the inner tube, its construction needs great care. I find the following mode of procedure gives good results. Select the pieces of tube for this instrument as free from curvature as possible. For the inner tube, a tube 12 mm., or rather more, in external diameter, and of rather thin glass, is drawn out, as for closing, until only a very narrow tube remains at C, the end of C is closed the area round C is carefully blown into shape, so that by melting off C the tube A will be left with a well-rounded end. A small bulb of glass is next blown on A at B. This bulb must be of slightly greater diameter than the contracted end E of the larger tube (II.), so that B will just fail to pass through E. The length from B to C must not be made greater than from E to G on the outside tube. The end at C is then to be cut off so as to leave a pin-hole in the end of A.
Fig. 19.
The outer tube (II.), whose diameter may be 5 or 6 mm. greater than that of A, is prepared by sealing a side tube on it at F, after previously contracting the end E. For this purpose the end E should be closed and rounded, and then re-heated and blown out till the bulb bursts. To ensure that the diameter of the opening is less than that of the tube, care must be taken not to re-heat too large an area of the end before blowing it out. It is very important that the cross section at E shall be in a plane at right angles to the axis of the tube.
Wrap a strip of writing paper, one inch in breadth, closely round the end of A at C till the tube and paper will only just pass easily into the mouth D of the outer tube, push the inner tube A, with the paper upon it, into D, and when the paper is entirely within D, withdraw A, and cautiously push the paper a little further into the outer tube. Insert A into DE through E, so that the bulb B is embraced by E. Close D with a cork. Ascertain that the paper does not fit sufficiently tightly between the two tubes to prevent the free passage of air, by blowing into the mouth K of A. Air should escape freely from E when this is done. Gradually bring the line of contact of B and E and the surrounding parts of the tube before a pointed flame, after previously warming them by holding near a larger flame, and rotate them before the flame so that the glass may soften and adhere. Then heat the joint spot by spot as usual. In blowing this joint, take care that the glass on each side of the actual joint is slightly expanded. It should present the form shown by the dotted lines in III. (these are purposely exaggerated, however). Finally, heat the whole joint between the lines JI till it softens, and simultaneously blow and draw it into its final shape as seen at III.
The side tube F should not be too near the end E. If, however, it is necessary to have them close together, the joint F must be very carefully annealed when it is made; it must also be very cautiously warmed up before the construction of the joint at H is begun, and must be kept warm by letting the flame play over it from time to time during the process of making the latter joint.