§ 40. WEIGHING OF RESIDUES AXD PRECIPITATES. 39 



particles of tlie precipitate that may have fallen on tlie 

 first piece of paper, roll the filter up, enclose it in a short 

 spiral on one end of a platinum wire that was weighed 

 with the crucible, hold it over the crucible, and set fire to 

 it ; by applying tlie charred filter to the flame of the 

 lamp two or three times it may be almost completely in- 

 cinerated ; finally, either let the ash and the wire drop 

 into the crucible and ignite the whole four or five minutes, 

 or until the ash is white, or, in case the filter-ash must be 

 kept entirely separate from the i)recipitate, let the two 

 drop into the hollow lid of the crucible, and ignite the 

 precipitate and ash separately. 



The glazed paper used above should be of a light color 

 if the precipitate is dark-colored, and vice versa, and the 

 whole operation should be performed in a place free froi:i 

 currents of air. 



c. If the quantity of the precipitate is very small, and 

 yet is of such a nature ns to be partly reduced to a lower 

 degree of oxidation if ignited with the filter, the ignition 

 may be performed as in a ; when it is completed, put a 

 piece of dry ammonic nitrate in the crucible, cover well, 

 and ignite again, but very gently at first. 



Ferric oxide or baric sulphate may be ignited in this 

 way when nothing better can be done. 



Sometimes, when a portion of the filter is very difficult 

 to incinerate completely, the combustion may be facilita- 

 ted by adding a little ammonic nitrate as above. 



After weighing, subtract the weight of the filter-ash, 

 which has been determined once for all for the particular 

 kind and lot of paper and size of filter used, by the incin- 

 eration of half a dozen or a dozen together, and dividing 

 the total weight of the ash thus obtained by the number 

 of filters burned. 



d. If the substance to be weighed cannot be ignited, a 

 filter should be previously thoroughly dried in the steam 

 or air-bath at the same temperature to which it is alter- 



