38 § 40. ANALYTICxVL MANIPULATION". 



continued, the liquid will be forced out through the mus- 

 lin filter ; on immersing the oi)en end of the longer tube 

 in a fresh quantity of the solvent, and removing the lamp, 

 this liquid will flow in. 



The solution may not be perfectly clarified in passing 

 through the linen filter, in which case it will have to be 

 filtered again through paper. 



To efiect more perfect filtration, a thick mat of gun- 

 cotton may be bound over the linen ; this layer of cotton 

 should not be anywhere less than 14 mm. thick. 



WEIGHING OF RESIDUES AND PRECIPITATES. 



40. When it is possible, residues or precipitates are ig- 

 nited before being weighed. 



This ignition may be performed in two ways. 



a. If the substance is not altered in its chemical com- 

 position by contact with burning organic matter, or at 

 the somewhat high temperature that is sometimes neces- 

 sary to effect the complete incineration of the filter, roll 

 the well-dried filter together around the precipitate, put 

 the whole in the previously ignited and weighed crucible, 

 cover and heat, at first very gently ; when the filter is 

 completely charred and no more smoke is given off", turn 

 the crucible on its side, lay the cover j^artly on the edge 

 of the crucible and partly on the triangle, and heat the 

 contents of the crucible until the ash is quite white. 



h. If the filter may not be burned in direct contact 

 with the precipitate, crush and work it gently between 

 the fingers over a sheet of glazed paj^er, to loosen the pre- 

 cipitate as much as possible, place the crucible on the 

 glazed paper, and empty the contents of the filter into it. 

 Put the crucible on the porcelain plate belonging to the 

 Bunsen's burner, open the filter on another piece of 

 glazed paper, fold its edges up so as to make a little tray, 

 with a soft feather carefully brush into this tray any 



