390 PiiOCEEDlNGrf OF THE AMEUICAN ACADEMY 



XVI. 



A METHOD OF FILTRATION V>Y MEANS OF EASILY 

 SOLUBLE AND EASILY VOLATILE FILTERS. 



By F. A. Goocn. 



Communicated March 11, 1885. 



TnE processes of analysis, in wliicli it is desirable to redissolve precip- 

 itates from the filter after washing, or to separate a mixed precipitate 

 into parts hy the action of" appropriate solvents, are many. 



When a complete solution is the ohject, and the precipitate yields 

 easily to solvents which do not alfect paper injuriously, the use of the 

 ordinary filter oilers no difficulty. When, however, precipitates are 

 to be treated with reagents which disintegrate paper filters, the case 

 is otherwise ; and the attempt to remove, by solvents, any individual 

 part of a mixed heterogeneous mass upon a filter, is always an uncer- 

 tain matter. As examples of cases of this sort, difl&cult to deal with, 

 we may take the solution of acid sodic titanate in strong hydrochloric 

 acid ; or, the purification of baric sulphate from included salts, by 

 digestion in strong hydrochloric acid ; or, the sejxiration of sulphides 

 which are soluble from those which are insoluble in alkaline sulphides; 

 or, the washing out of free sulphur from precipitated sulphides by 

 means of carbon disulphide ; or, the separation of calcic and baric 

 sulphates by the action of sodium hyposulphite. In cases of this 

 nature it is often convenient to make use of the asbestos filter which 

 I have previously described ; * but this sometimes has its disadvan- 

 tages. Thus, to recur to the examples just cited, acid sorliura titanate 

 may be filtered and washed upon an asbestos filter, and felt and pre- 

 cipitate treated together with hydrochloric acid, but it will be impos- 

 sible to determine when solution is efTected because of the floating 

 asbestos ; and in separating the sulphides it would be necessary to 

 know the weight of the asbestos felt, since it must be weighed finally 

 with the insoluble sulphides, unless removed by a special treatment 

 which involves the solution, filtration, and repreci[)itatiou of the 

 latter. 



♦Tliese rroceedings, Vol. XIII. p. 342. 



