t _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]

[13] For a method of joining soda glass to lead glass, see p. 81.

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.

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.

But in most cases one or other of the following air-tight joints can be
employed, and will be found to be very convenient:--

=Mercury Joints.=--The simplest form of mercury joint is shown at Fig.
30. _A_ and _B_ are the two tubes which are to be connected. A larger
tube or cup _F_ is attached to _A_ by the india-rubber tube _E_, and
placed on _A_ so that the end of _B_ may be brought into contact with
_A_ at _C_, and connected to it by a well-fitting piece of india-rubber
tube _C_. The cup _E_ is then brought into the position shown in Fig.
30, and mercury is introduced till the india-rubber tube at _C_ is
covered. As mercury and glass do not come into true contact, however,
such a joint, though said to give good results in practice, is not
theoretically air-tight, for air _might_ gradually find its way between
the liquid and the glass. By covering the mercury with a little
sulphuric acid or glycerine the risk of this occurring may be removed.
The same result may be attained by the use of glycerine in place of the
mercury in the cup _F_; but glycerine is less pleasant to work with than
mercury.[14]

[14] If the india-rubber tube _C_ be secured by wires, iron wire, not
copper wire,