We have repeatedly heard of asbestos being a filter medium and of the interesting experiments that have seemed to show its superiority to most other mediums in fineness of interstices. The question, however, remains: Can it be kept clean? says the Sanitary Era, Its incombustible nature suggests the possibility of readily purifying it by heat; but whether its filtering property and form would stand the ordeal unimpaired, is a matter that nobody seems to have thought of investigating. We described under the heading of "filtering-material" this filter medium, which already forms part of some filters.

It is claimed that liquids filter altogether too slowly through asbestos. In many cases, a very finely divided asbestos is desirable.

The grade of asbestos to use in connection with a filter for the mineralwater trade is a matter of importance; if coarsely ground it will certainly be an excellent filter medium, when used instead of sand, and, in connection with animal charcoal, give fair results.

Asbestos filters are very useful in cases where the liquid to be filtered is of a caustic or strongly acid nature.

The kind of asbestos to use is a matter of great importance also. In commerce we find the Canadian, the Italian, the Australian.

This last is less flexible than the other two, and consequently the fibres do not felt together and pack as closely on the perforated plate. Hence, liquids filter more rapidly, and the Australian is, on this account, preferable to the other two kinds. It is claimed that the Canadian asbestos is the most soluble in acids, but the assertion is not verified.

Whatever may be the kind of asbestos used, the following is a process for obtaining with little trouble a quantity of the pulp in a fit state for filtration, and as this pulp will also be a very useful filtrant in the bottlers' laboratory, we append the direction for its preparation, as given in the National Bottlers' Gazette, A coarse brass sieve is placed over a sheet of paper, and a handful of asbestos is rubbed pretty roughly over the sieve-cloth. This breaks it up in such a way that the smaller fragments pass through the meshes, and are deposited on the paper underneath. After a while, the portion which remains on the sieve-cloth is collected in one bundle, and rubbed again in the same manner, and the operation is repeated until a sufficient quantity has gone through.

As to the coarseness of the mesh to use, we may say that we have used No. 10 sieve (ten openings to the inch) with satisfactory results. The sieve is best placed bottom up, so as to leave plenty of room under the cloth.

The next operation is to free the sifted material from dust and from the finest particles. This is easily accomplished by placing the asbestos, as obtained above, over another sieve of finer mesh (about No. 25 or No. 30), and stirring it while water is poured over the sieve. The first water which passes through is quite milky, but it gradually becomes clearer as the washing is continued. The washed asbestos is then put in a beaker glass, and boiled for about half an hour with strong hydrochloric acid (about one part of fuming H C l to four parts of water).

The pulp, after this treatment, is poured over a perforated platinum plate placed in a funnel, and washed with distilled water until no acidity is shown by litmus paper. The pulp is then taken out of the funnel and strongly heated in a platinum dish. After letting it cool sufficiently, it may be placed in a wide-mouthed bottle for future use.

An asbestos filter, called the Filter Rapide, contained, as far as we could learn, thus prepared asbestos pulp.