When the end of the tube presents the appearance shown in the diagram D, and the mass of glass at d is small, the small lump that remains must be removed by heating it till it softens, and gently blowing with the mouth, so as to round the end and distribute the glass more regularly, as shown in E. The whole end, from the dotted line e, must then be heated with constant rotation in the flame. If this final heating of the end e be done skilfully, the glass will probably collapse and flatten, as at F. The end must then be gently blown into the form shown at G.
If a flat end to the tube be desired, the tube may be left in the condition shown by F, or a thin rounded end may be flattened by pressure on a plate of iron.
If a concave end be wished for, it is only necessary to gently suck air from the tube before the flattened end has become solid.
Fig. 9.
In each case, immediately after the tube is completed, it must be closely wrapped in cotton wool and left to cool. With good lead glass this last process, though advantageous, is not absolutely necessary; and as glass cools slowly when enveloped in cotton wool, this precaution may frequently be neglected in the case of apparatus made from lead glass.
In order to draw out tubes for sealing, close to one end, and thus to avoid waste of material, it is a good plan to heat simultaneously the end of the glass tube A which is to be sealed, and one end of a piece of waste tube E of about the same diameter, and when they are fused to bring them together as at DD ([Fig. 9]). E will then serve as a handle in the subsequent operations on A. Such a rough joint as that at D must not be allowed to cool too much during the work in hand, or E and A may separate at an inconvenient moment. Or the glass at the end of the tube may be pressed together to close the tube, and the mass of glass may be seized with a pair of tongs and drawn away.
Choking, or Contracting the Bore of a Glass Tube.—If it be not desired to maintain the uniformity of external dimensions of the tube whilst decreasing the diameter of the bore, the tube may be heated and drawn out as described in the description of sealing tubes on [pp. 32]-[35]. This may be done as shown at A or B in [Fig. 8], according to the use to which the contracted tube is to be put.
