346 PROCEEDINGS OF THE AMERICAN ACADEMY 



pump, be passed through a gauze cone coated with asbestus felt, which 

 exposes a filtering surface of twenty-four square centimetres (nearly 

 the same as that of a paper-filter, eight centimetres in diameter, when 

 folded in the ordinary manner), with ease at the rate of a litre per 

 minute. 



When the filtering surface is less, the rapidity of filtration is, of 

 course, somewhat diminished, but always exceeds that of paper of the 

 same dimensions. When the felt is deposited upon gauze, the layer 

 may be surprisingly thin and yet be efficient enough for all ordinary 

 purposes. If the layer of felt be quite thick, the filtrate from baric 

 sulphate freshly precipitated in the cold, may be made to pass through 

 clear. But the great superiority of asbestus felt lies in its constancy 

 of weight, whether dried at high or low temperatures, the rapidity with 

 which it may be safely and completely dried, and its refractoriness as 

 regards acids (excepting hydrofluoric acid) and alkalies. These advan- 

 tages appear most prominently in processes which involve the separa- 

 tion and desiccation : — 



First, of precipitates which (like K,PtCl,;, [HgJCl,) must be dried at 

 low temperatures, on account of a tendency to decompose spontane- 

 ously at high tediperatures ; 



Secondly, of precipitates which (like .SU.So, PbSO^, ZnCOJ have usu- 

 ally been dried slowly and tediously at low temperatures, or have 

 been separated from the filter before ignition, to avoid the danger of 

 reduction by heating in contact with carbon ; 



TIdrdhj, of precipitates whicli (like (NH,),Mg,P,,0^, (NH,),Mn.,P._,0^) 

 may be ignited in contact with carbon, but which make its complete 

 combustion difficult. 



In all cases, the time required to dry or ignite precipitate and filter 

 is a minimum, inasmuch as heat may be applied as soon and to as high 

 a degree as the precipitate itself will permit. Even in jirocesses in 

 which paper filters find their most convenient application, paper has 

 generally no superiority over asbestus felt. 



To illustrate the cohesiveness and refractory nature of the felt, I 

 append the account of a single experiment. All of the liquids used 

 had been previously filtered, excepting the distilled water with which 

 the filter was washed, after the passage of each liquid. This precau- 

 tion was considered unnecessary in the case of the water; but, if it had 

 been taken, the slight increase of weight which was observed during 

 the experiment would probably have been prevented. 



