Fig. 26.
In order to secure a thick end to the point of the tube a, about an inch or so of the tube near the contracted part should be warmed a little, if it is not already warm, at the moment of finally sealing it; the contraction of the air in the tube, in consequence of the cooling of the warm tube, will then ensure the glass at a running together to a solid end when it is melted in the flame.
If it will be necessary to collect a gas produced during a chemical action from such a tube, make the contracted end several inches long, and bend it into the form of a delivery tube. It will then be possible to break the tip of this under a cylinder in a trough of liquid.
In order to explain the construction of apparatus consisting of several parts, it will be sufficient to take as examples, two very well-known instruments, and to describe their construction in detail. From what is learned in studying these, the student will gather the information that is wanted.
Fig. 27.
1. To make Hofman’s Apparatus for the electrolysis of water ([Fig. 27]).
Take two tubes about 35 cm. in length, and 14 mm. in diameter for AA, join taps TT to the end B of each of them, draw out the other end, as shown at D, after sheets of platinum foil with wires attached to them[12] have been introduced into the tubes, and moved by shaking to BB. Then allow the platinum wires to pass through the opening D left for the purpose, and seal the glass at D round the platinum as at E. Pierce the tubes at JJ, and join them by a short piece of tube K, about 14 mm. in diameter, to which the tube T, carrying the reservoir R, has been previously united. R may be made by blowing a bulb from a larger piece of tube attached to the end of T. The mouth M of the reservoir being formed from the other end of the wide tube afterwards. One of the taps can be used for blowing through at the later stages. Each joint, especially those at JJ, must be annealed after it is blown. Some operators might prefer to join AA by the tube K in the first instance, then to introduce the electrodes at E and D. In some respects this plan would be rather easier than the other, but, on the whole, it is better to make the joints at JJ last in order, as they are more apt to be broken than the others during the subsequent manipulations.