OF ARTS AND SCIExNCES. 391 



It is to meet cases like these that I liave soufrht a filter which, in 

 the reversal of the ordinary mode of separating filter and precipitate, 

 should dissolve easily in solvents which do not affect the ordinary 

 precipitates met with in analysis. The material whicli seems best 

 suited to the case — light and fluffy, capable of making secure filters 

 of any desirable degree of porosity, suificiently insoluble in water and 

 aqueous solutions of salts, alkalies, and acids (excepting strong sul- 

 pluiric, strong nitric, and glacial acetic acids), easily soluble in naphtha, 

 benzol, carbon disulphide, ether, boiling alcohol, and essential oils, 

 and not too costly — is anthracene. 



The mode of preparing and using the filter is simple. Anthracene 

 is slightly moistened with alcohol to make it miscible with water, 

 diluted to the right consistency, and applied to the same apparatus, 

 and in tiie same way, as the emulsion of asbestos whicli is employed 

 in making asbestos felts. That is to say, enough of the emulsion in 

 water to form a layer of the proper thickness is poured into a perfo- 

 rated crucible which is held tightly in a packing of rubber tubing 

 stretched over a funnel fitted in the usual manner to a vacuum-flask 

 or receiver. After washing with water the filter is ready for use. If 

 the felt happens to be too coarse for the use of the moment, it may be 

 made as close as need be by coating the felt first deposited with a 

 finer emulsion, made by dissolving anthracene in hot alcohol and 

 precipitating with water. When voluminous precipitates are to be 

 filtered, the large perforated cone described in the former paper, to 

 which I have referred, may be substituted with advanta^je for the 

 crucible ; or Cooke's improved form * of Carmichael's process of 

 reverse filtration may prove most useful. In using the cone it is well 

 to apply the anthracene in a thick layer. 



To remove the anthracene filter from a precipitate, it is only neces- 

 sary to act with the proper solvent. It is usually convenient to stand 

 the crucible containing precipitate and felt in a small beaker, add 

 enough of the solvent, and gently warm until the anthracene dissolves. 

 On the addition of water, or the reagent to work upon the precipitate, 

 the solution of anthracene floats, and nothing remains to obstruct or 

 obscure the action. If the precipitate dissolves entirely, the solution 

 of anthracene may be separated from the aqueous solution by simply 

 pouring the fluid upon a filter previously moistened with water, when 

 the solution in water runs through, and the anthracene and its solvent 

 remain and may be washed indefinitely with water. 



* These Proceedings, Vol. XII. p. 124. 



