150 § 91. SPECIAL METHODS OF AXALYSIS. 



combustion ; to eliminate this, one of two courses may 

 be followed. 



1. Exhaust the mixture of ash and coal with hot water, 

 collect the insoluble part on the filter, wash, dry, and ig- 

 nite it ; the coal will generally be found to burn much 

 more readily after this treatment, and the ash can more- 

 over be heated to a higher temperature than before with- 

 out fear of loss. Add the ash so obtained to the aqueous 

 extract and washings, evaporate to dryness, ignite gently, 

 and weigh. 



2. Or, weigh the mixture of ash and unconsumed car- 

 bon, determine carbonic acid (d) in the whole or a por- 

 tion of it, collect what is insoluble in the nitric acid in the 

 determination of the carbonic acid, on a dried and weigh- 

 ed filter, wash it well, dry at 110° C, weigh, ignite until 

 the carbon is completely burned, and weigh again. The 

 loss of weight gives the unburned carbon in that portion 

 of the original ash taken ; calculate the amount of coal 

 for the whole quantity of the original mixture of ash, 

 including carbonic acid and coal, and deduct it from the 

 same. 



d. A portion of the carbon in the organic part of the 

 substance ignited may remain behind in combination with 

 the metallic oxides as carbonic acid ; sincQ. this does not 

 properly belong to the ash or inorganic part of the sub- 

 stance, it should be determined and deducted from the 

 total weight of the ash. 



For this purpose estimate the carbonic acid (§ 60) in a 

 portion of the ash, or the whole of it, according to the 

 quantity in hand, calculate the amount for the whole 

 quantity of the ash, if only a portion was used for the 

 analysis, and deduct it from the same. 



A substance may, however, contain a notable quantity 

 of carbonic acid before ignition, as, for example, a soil 

 with carbonate of lime in it. In this case the ignited 

 residue should be moistened with ammonic carbonate. 



