Fig. 33.
Vacuum Taps.—It is not necessary to enter into a description of the construction of ordinary glass taps, which can be purchased at very reasonable prices. It may be remarked here, however, as a great many of them are very imperfectly ground by the makers, that they may easily be made air-tight by hand-grinding with camphorated turpentine and fine emery, finishing with rotten-stone. A well-ground tap, which is well lubricated, should be practically air-tight under greatly reduced pressure for a short period; but when it is necessary to have a tap which absolutely forbids the entrance of air into apparatus, one of the following may be employed:—
Fig. 34.
(1.) Mr. Cetti’s Vacuum Tap ([Fig. 34]): This tap is cupped at A and sealed at B, and the cup A is filled with mercury when the tap is in use, so that if, for example, the end C be attached to a flask, and D to an apparatus for exhausting the flask, it will be possible to close the flask by turning off the tap E, and if no air be allowed access through D, the vacuum produced in the flask at C cannot be affected by air leaking through the tap at A or B.
A passage F must be drilled from the bottom of the plug E to meet G, in order that when the plug is in position no residue of air shall be confined within B, whence it might gradually leak into any apparatus connected to it.
It is obvious, however, that this tap does not protect a flask sealed to C from the entrance of air through D, which, in fact, is the direction in which air is most likely to effect an entrance. When using one of these taps as part of an apparatus for supplying pure oxygen, I have guarded against this by attaching a trap ([Fig. 33]) to the end D, C being joined to the delivery tube from the gas-holder. The structure and mode of action of the trap are as follows:—
