Another very important use to which it is now being applied is in the manufacture of filters. These are specially useful where the liquid to be filtered is of a caustic or strongly acid nature, or where the filter with residue is to be ignited without consuming the filter, or where the residue is to be subsequently dissolved off the filter by acids or other solvents. In many cases a very finely divided asbestos is desirable. This is accomplished by a process recently patented in Germany by Fr. Breyer, of Vienna. The asbestos is first coarsely ground, and then mixed with some granular crystalline carbonate, which must be soluble in acids. The carbonate should possess a hardness between 3 and 4, 5, according to the mineralogical scale. The mixture is ultimately ground together in a mill. Afterwards the mass is treated with an acid until the carbonate has been dissolved out. The escaping carbonic gas causes the asbestos fibres to be loosened and disintegrated from each other so as to render the mass porous. Of course it must be thoroughly washed with water before being used.
Again, in the purification of foul gas, as well as for ventilating and deodorizing man-holes for cesspools, sewers, &c., its use is found to be unsurpassed by any other known material.
Mr. Boyd, in the paper so often referred to, says in regard to this, that he was some time previously asked to supply asbestos yarn spun in such a way as to have good capillary action, and, on making inquiry, found it was to be used for the above purpose. In describing the mode of using it, he says that there is placed over the opening rising from the sewer a hood of galvanized wire, interlaced with this asbestos yarn, the ends of the yarn dipping into a receptacle filled with liquid disinfectant, which, as they become saturated, form a disinfecting screen, through the meshes of which the gases rise, and in their passage through are purified and rendered innocuous. The system hitherto previously adopted for deodorizing sewer gas has been to cause it to rise through charcoal, but it is found that the impurities soon clog this up, and simply prevent the passage of the gas, whereas in the arrangement just mentioned (which is that of Messrs. Adams & Co., of York), the gas rises freely, and is perfectly deodorized.