Fig. 31.
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]
When sulphuric acid is to be employed in such a joint, or when for any other reason the use of an india-rubber tube is undesirable, the joint may consist of a hollow stopper B ([Fig. 31]), made of glass tube, and ground to fit the neck of a thistle funnel A. A and B are joined respectively to the pieces of apparatus to be connected, and connection is made by placing B in position in the neck of A; the joint is made air-tight by introducing mercury with strong sulphuric acid above it into the cup A. The joint may be rendered air-tight by introducing sulphuric acid only into the cup. But this plan must not be adopted if the interior of the apparatus is to be exhausted, as sulphuric acid is easily forced between the ground glass surfaces by external pressure. Mercury, however, will not pass between well-ground glass surfaces, and is therefore to be employed for connecting apparatus which is to be exhausted, and, if necessary, protected by a layer of strong sulphuric acid to completely exclude air.
Fig. 32.
Tubes placed horizontally may be joined by a glycerine or mercury joint such as is shown in [Fig. 32]. The two tubes A and B are joined as before by an india-rubber connection C, or one may be ground to fit the other, and the joint is then enclosed within a larger jacketing-tube D, with a mouth at F, which is filled with glycerine or mercury. D is easily made by drawing out both ends of a piece of tube, leaving them large enough to pass over the connection at C, however, and piercing one side at F.
