§ 39. FiLTRATiox; bunsen's process, 33 



In washing precipitates on the filter, the washing-bottle 

 is an indispensable aid. This consists simply of a flask 

 of a capacity of 150-1000 c.c, according to the purpose 

 for which it is to be used, closed by a good cork that is 

 pierced Avith two holes ; through one of these holes passes 

 a glass tube, 8 or 10 cm. long, that extends just beyond 

 the coik on tlie inside, and, outside, is bent at an angle 

 of about 110° ; the tube that passes through the other 

 hole extends nearly to the bottom of the flask, and, out- 

 side, is bent at an angle of about 70°, and drawn out to a 

 small jet at the end; water in the flask is forced out at 

 this jet on blowing air in at the mouth of the shorter 

 tube. 



Each portion of water with yrhich a precipitate on the 

 Clter is washed should be allowed to pass through com- 

 pletely before another is added, and the precij^itate should 

 be stirred up as much as possible by the jet from the 

 wash-bottle with .each fresh addition. 



Insoluble residues and j^recipitates must be washed, 

 particularly in quantitative operations, as long as the wash- 

 water carries oflf any notable quantity of matters in solu- 

 tion ; the washings are tested by evaporating a drop to 

 dryness on platinum foil, to see if any residue is left, or 

 by a chemical test, as, for example, when washing a pre- 

 cipitate of baric sulphate that was formed by adding 

 baric chloride to a solution of a sulphate; r.s long as any 

 of the soluble chloride remains in the pores of the filter, 

 or adheres to the precipitate, and is taken up by the 

 Vv'ater, tlie Avashings will give the usual reaction for 

 clilorine with argentic nitrate (§ 63). 



When the contents of the filter are to be weighed or 

 ignited, dry the whole together in the drying-chamber or 

 air-bath, with the funnel well covered with filter paper. 



K A metliod lately devised by Bunsen {Annalen der 

 Chemie, 148, 270. American Journal of Science and 

 Art, 2d Series, 4-7, 321) for iiicreasing the rapidity of fil- 



