Fig. 14.
When small tubes, or tubes of fine bore, are to be joined, in order to prevent the fused glass from running together and closing the tube, it is a good plan to border and enlarge the ends that are to be united, as at A ([Fig. 14]). Some glass-blowers prefer to border all tubes before uniting them.
When a narrow tube is to be joined to one that is only slightly wider, expand the end of the narrow tube till it corresponds in size to the larger tube. If the tube be too narrow to be enlarged by inserting a charcoal cone, seal one end and pierce it as directed (on [p. 37]).
For joining small thin-walled tubes Mr. Crookes recommends the use of a small Bunsen flame.
Fig. 15.
In welding pieces of lead glass tube, take care that the heated glass is perfectly free from reduced lead at the moment when the two ends of viscous glass are brought into contact.