Fig. 37.
If A has to be removed, C may be converted into a mercury joint pro tem. by letting a little mercury from the upper cup fall into the tube and cover d, the tap being closed. This mercury must be removed by a fine pipette in order to use the tap again. It should be noted, however, that though external air cannot enter by way of the ground glass joints, there is no absolute protection against the passage of air between A and C, or vessels joined to A and C, even when the tap is closed. The passage of air from A to C depends upon the grinding and lubrication of the joint at C.
Lubricating Taps.—For general purposes resin cerate answers very well. In special cases burnt india-rubber, or a mixture of burnt india-rubber and vaseline will answer well, or vaseline may be used alone. Sulphuric acid and glycerine are too fluid. When a lubricant is wanted that will withstand the action of ether, the tap may be lubricated by sprinkling phosphorus pentoxide upon it, and exposing it to air till the oxide becomes gummy. The joint must then be protected from the further action of the air if possible. For example, if a safety tap be used the cup may be filled with mercury.
Air-Traps.—In [Fig. 33], [p. 66], an air-trap (J) is shown. An air-trap is a device for preventing the mercury supplied to Sprengel pumps, etc., from carrying air into spaces that are exhausted, or are for any reason to be kept free from air. Figs. 36 and 37 give examples of air-traps. In the simpler of the two ([Fig. 36]) mercury flowing upwards from C that may carry bubbles of air with it passes through the bulb A, which is filled with mercury before use.[16] Any air which accompanies the mercury will collect at a, the mercury will flow on through b. So long as the level of the mercury in A is above b, the trap remains effective.
In the trap shown by [Fig. 37], the tube d, which corresponds to b in [Fig. 36], is protected at its end by the cup E. E prevents the direct passage of minute bubbles of air through d. This trap, like the other, must be filled with mercury before it is used, and it will then remain effective for some time.
[11] Large tubes may also be bent by rotating a sufficient length of the tube in a large flame till it softens, and bending in the same manner as in the case of smaller tubes, and after filling them with sand, closing one end completely, and the other so that the sand cannot escape, though heated air can do so.
[12] Red-hot platinum welds very well. The wire may be joined to the sheet of foil by placing the latter on a small piece of fire-brick, holding the wire in contact with it at the place where they are to be united, directing a blow-pipe flame upon them till they are at an intense heat, and smartly striking the wire with a hammer. The blow should be several times repeated after re-heating the metal.
[13] For a method of joining soda glass to lead glass, see [p. 81].
[14] If the india-rubber tube C be secured by wires, iron wire, not copper wire, should be employed.