USED IN CHEMISTRY. 7 



Washing is used for procuring powders of an 

 uniform fineness, much more accurately than by 

 means of the sieve ; but it can only be used for 

 such substances as are not acted upon by the fluid 

 which is used. The powdered substance is mixed 

 ■with water, or other convenient fluid : the liquor 

 is allowed to settle for a few moments, and is then 

 decanted off; the coarsest powder remains at the 

 bottom of the vessel, and the finer passes over 

 with the liquor. By repeated decantations in this 

 manner, various sediments are obtained, of different 

 degrees of fineness : the last, or that which re- 

 mains longest suspended in the liquor, being the 

 finest. 



Filtration is a finer species of sifting. It is 

 sifting through the pores of paper, or flannel, or 

 fine linen or sand, or pounded glass, or porous 

 stones, and the like ; but it is used only for separ- 

 ating fluids from solids, or gross particles that 

 may happen to be suspended in them, and not chcr 

 mically combined with the fluids. Thus salt water 

 cannot be deprived of its salt by filtration j but 

 muddy water will deposit its mud. No solid, even 

 in the form of powder, will pass through the above- 

 mentioned filtering substances : hence if water or 

 other fluid, containing sand, insects, mud, &c. be 

 placed in a bag or hollow vessel made of any of 

 those substances, the sand, &c. will remain upon the 

 filter, and the liquor will pass through, and may be 

 received clear in a vessel under it. Unsized paper 

 is a very convenient substance for making filters 

 for chemical purposes. It is wrapped up in a 

 conical form, and put into a glass funnel, which 

 serves to strengthen the paper and support the 

 weight of the fluid when poured into it. 



Decantatioti.iS often substituted, instead of fil- 

 B 4 



