Fig. 29.
Modes of combining the Parts of Heavy Apparatus.—It is often necessary to connect pieces of apparatus which are too heavy to be freely handled before the blow-pipe, and which, therefore, cannot be welded together as described on [p. 39], by some more effective method than the ordinary one of connecting by india-rubber tubing. For example, apparatus which is to be exhausted by a Sprengel air-pump must be attached to the pump by a joint as perfectly air-tight as can be obtained. This, indeed, often may be done by welding the apparatus to be exhausted to the air-pump before the blow-pipe. But such a method is open to the obvious objection that it is very troublesome to connect and disconnect the parts as often as may be necessary, and that there is some risk of accidental breakages. Nevertheless it may be done on occasion, especially if there be no objection to the use of the flexible spiral tubes already alluded to. When the use of a spiral connecting-tube is not admissible the difficulty is considerably increased. For example, the author has lately required to attach an ozone generator, of the form shown by [Fig. 19], which previously had been cemented into a heavy copper jacket, to a pressure-gauge rigidly fixed to a support, and of considerable size. The employment of a flexible spiral connection was prohibited by the fact that it was necessary that the volume of the connecting-tube should be but a small fraction of that of the ozone generator, a condition which compelled the use of a tube of almost capillary bore, and of inconsiderable length. At the same time the frailness of such a connection made it necessary to fix the generator and pressure-gauge rigidly to their supports, in order to avoid the possibility of breakage by slight accidental movements of either of them, and it was obviously necessary to fix the pieces of apparatus in their final positions before joining them, lest the fine tube which connected them should be fractured during adjustment. The possibility of a strain being caused by the contraction that would occur during the cooling down of the joint last made had to be provided for also. The desired object was effected as follows. In [Fig. 29] A represents a section of the ozone generator at the point where the tube to connect it to the gauge was fixed. B represents the top of the gauge, with the side tube C, which was to be connected with that from A, viz. D. The ends of C and D were expanded as shown at D (by melting them and blowing them out), so that one of them, made rather smaller than the other, could be overlapped by the larger one. A and B being rigidly fixed in their final positions, with C and D in contact, as shown in the figure, all openings in the apparatus were closed, except one, to which was attached an india-rubber blowing-bottle by means of a tube of india-rubber long enough to be held in the hand of the operator, and to allow him to observe the operation of joining the tubes at D. When everything was in readiness, a very small-pointed flame from a moveable blow-pipe held in the hand was directed upon the glass at D till it melted and the two tubes united. To prevent the fine tube when melted from running into a solid mass of glass, and so becoming closed, a slight excess of pressure was maintained inside the apparatus during the operation by forcing air into it with the india-rubber blower from the moment at which C and D united. A point of charcoal was kept in readiness to support the softened glass at D in case it showed any tendency to fall out of shape.
The V-tube at C served to prevent the subsequent fracture of the joint in consequence of any strain caused by the contraction of the glass in cooling.[13]
It is not difficult to connect several pieces of apparatus successively in this manner, nor is this method only useful in such cases as that just described. Pieces of apparatus of great length and weight may be joined in a similar manner, irrespective of the size of the tubes to be united.
Fig. 30.
The ends to be joined, prepared as before, so that one slightly overlaps the other, must be held firmly in contact by clamps, and heated in successive portions by a blow-pipe held in the hand of the operator, each patch of glass being re-heated and gently blown, after a rough joint has been made. Finally, a larger flame may be used to heat up the whole joint for its final blowing. It is important to place the apparatus so that the operator has free access to it on all sides. A revolving table might be employed. An assistant to work the bellows is necessary. Or, better still, air may be admitted to the blow-pipe from a large gas-bag placed in some convenient position.
