478 PRACTICAL ORGANIC CHEMISTRY tube all the time the above operation is in progress, and the risk of any carbon dioxide or water vapour getting into the tube from the air is thus very small. The absorption tubes R and S are next joined together by a 20 mm. length of impregnated rubber tubing, the side-arms forming a glass-to-glass join inside the rubber; the side-arm at the "non-absorbing" end of the tube R being attached to the side-arm of the "absorbing" end of the tube S. The exit of the tube S is now attached to the guard tube V in the usual way and the pair of tubes finally attached by a 15 mm. piece of impregnated tubing, and glass-to-glass joint, to the beak of the combustion tube. As short a piece of tubing as is compatible with a gas-tight join is used for this connection so that as much as possible of the beak of the combustion tube and of the side-arm of R may be heated directly by the heating hooks Q attached to the thermostatic mortar. These hooks are next placed in position, both initially being
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