§ 38. PRECIPITATION^. 29 



lution to dryness in a porcelain dish on the water-bath, 

 dry the residue thoroughly at a temperature a little above 

 100° C, transfer it to another dish with the aid of a 

 spatula, rinse the porcelain dish with a little water into 

 the crucible in which tlie residue is to be finally ignited, 

 evaporate these washings to dryness, then ignite the dry 

 residue, obtained above, in small portions at a time, and 

 finally rinse the dish that contained it into the crucible, 

 with the aid of a little finely powdered ammonic chloride, 

 and ignite again. The dish with the residue should be 

 kept in the desiccator wiiile waiting for the ignition. 



PRECIPITATION. 



38 1 Precipitation is usually resorted to in order to sej)- 

 >arate certain substances from others in the same solution, 

 or simply from the solution itself; it consists in adding 

 some reagent to the solution, which causes the substance 

 or substances in question to enter into an insoluble form. 

 The operation is usually performed in beakers, because, 

 from these, the 2:>recipitate is more easily transferred to 

 the filter. 



Care must be taken not to use too large an excess of 

 the precipitant, and yet there must be no doubt at all that 

 enough has been added ; if the precipitate does not settle 

 speedily, so that the efiect of the addition of a few more 

 drops of the reagent can be observed, a small portion of 

 the mixture should be throwm on the same filter that is 

 finally to receive the wdiole of the precipitate, and the 

 necessary test can be applied to the filtrate ; this small 

 portion that has been separated from the main part of the 

 liquid should then be mixed with it again, before more of 

 the precipitant is added. 



The solution and the reagent should always be well 

 mixed by stirring, and, in most cases, the solution should 

 be so dilute that, when the precipitate settles, it will not 



