32 § 39. ANALYTICAL MANIPULATION. 



Most liquids may be filtered raucli more rapidly when 

 hot, and many precipitates are much less liable to pass 

 through the filter, or to choke it up, when formed in nearly 

 boiling hot solutions by hot reagents. 



When possible, it is best to let the solid matter settle 

 to the bottom of the vessel containing the mixture of 

 liquid and precipitate, then to decant as much as possible 

 of the clear, supernatant liquid on the filter, pour fresh 

 distilled water over the contents of the beaker, stir well, 

 and perhaps heat almost to boiling, let the precipitate set- 

 tle, and decant the liquid again ; this may be repeated. 'a 

 number of times before putting the solid substance on the 

 filter. 



If the precipitate is to be dissolved without weighing 

 or ignition, it is generally best to wash it altogether by 

 decantation, and then to pour the solvent over the filter 

 through which the decanted liquid was passed, and collect 

 it in the beaker containing the main portion of the ^vashed 

 precipitate ; the precipitate may then be digested with 

 the reagent if necessary, and, afterwards^ the filter well 

 washed out with water, that is added to the solution just 

 made ; in this way we may avoid any considerable dilu- 

 tion of the solvent before it has had time to act on the 

 substance to be dissolved. If the solvent is one that, in 

 its concentrated state, would attack the paper, it may be 

 poured at once over the precipitate in the beaker, while 

 another portion may be diluted somewhat, and passed re- 

 peatedly through the filter, to take up the small quantity 

 of the substance on that. 



The thorough washing of precipitates and residues, 

 that is so essential in quantitative analysis, and is often 

 not unimportant in qualitative work, may sometimes be 

 greatly facilitated by this process of decantation, particu- 

 larly if the solid is one that settles readily ; but if Bun- 

 sen's process of filtration is followed, decantation may be 

 dispensed with. 



