To tighten joints where a leakage is visible or a hissing sound is heard, use a cement composed of Natron-water glass (silicate of soda), which can be bought in commerce as a syrup-like mass; mix thoroughly with some powdered chalk (whiting). This cement hardens very quickly and closes any leakage of joints.

Another cement for resisting sulphuric acid, even at a boiling heat, may also be made by melting caoutchouc at a gentle heat, and adding, with constant stirring, from six to eight per cent, of tallow. Then mix therewith enough dry slaked lime to make the whole the consistency of soft paste; finally add thereto about twenty per 'cent, of red lead, whereby the mass immediately sets hard and dry, and must therefore be quickly used.

A solution of caoutchouc in twice its weight of linseed oil, aided by heating, and the addition of an equal weight of pipe clay, yields a plastic mass which will likewise resist most acids.