Fig. 17.
Tubes may also be joined to openings made in the sides of globes or flasks; great care must be taken, however, especially if the walls of the globe be thin, to secure that the tube is well attached to the mouth of the opening when the melted ends are first brought into contact, for, with thin glass, any hole that may be left will probably increase whilst the joint is being blown into shape, owing to cohesion causing the glass to gather in a thickened ring round an enlargement of the original opening.[8]
In order to unite a tube of soda glass to a tube of lead glass, the end of the soda glass tube must be carefully covered with a layer of soft arsenic glass.[9] This must be done so perfectly that when the ends to be united are brought together the lead and soda glass are separated by the enamel at every point.
To Seal a Tube inside a Larger Tube or Bulb.—Suppose that an air-trap (3 of [Fig. 18]) is to be constructed from a small bulb (A) blown on a glass tube (1).
Fig. 18
Either cut off the tube close to the bulb at B, or better, remove the end by melting the glass and pulling it away from B, and then pierce A at B, No. 2, by heating the glass there and blowing out a small bulb as described under Piercing.
Prepare a tube (4) drawn out at E with a bulb blown at D. Insert E into the opening B, press D well against the mouth B and slowly rotate before the blow-pipe till D adheres to B. Then heat and blow the joint spot by spot as in other cases, taking care that the glass is blown out on each side of the joint; lastly, heat the whole joint between aa, and blow it into its final shape.