§ 91. IXCINERATION. 149 



h. If the substance in solution is liable to form hard 

 clumps on drying that retain water mechanically enclosed, 

 and yet the residue cannot be heated much above 100 C, 

 mix it with ^ 1^ or ' |g of its weight of rather finely pulver- 

 ized crystallized gypsum, or of pure ignited baric sul- 

 phate, that has been artificially prepared, or with 3 or 4 

 times its weight of well-washed fine sand. If gypsum is 

 used, it should be tested beforehand, to see whether it 

 loses any weight at 100°. The mixture should be well 

 stirred as the evaporation approaches dryness. Heat the 

 residue at 100° in the usual manner as long as it loses 

 weight. 



INCINERATION, OR ESTIMATION OF ORGANIC MATTER. 



91. The dried residues obtained in the preceding sec- 

 tion are often examined for organic matter by ignition 

 until this matter is burned away, or incineration. 



a. The ignition is performed in a platinum dish or cru- 

 cible at as low a temperature as possible, with provision 

 for the access of air to the substance along the surface of 

 the lid of the crucible, as directed for the incineration of 

 filters (§ 40) ; or a piece of platinum foil may be bent so 

 as to rest on the bottom of the dish and on the rim, and 

 extend some distance beyond the latter. 



b. A portion of the original solid substance may be in- 

 cinerated at once in the muffle furnace, as described in 

 § 123, c, under preparation of the ash of plants for analy- 

 sis. Then, on subtracting from the loss of weight in this 

 trial the amount of water in the quantity of substance 

 taken, as may be calculated from the results of the esti- 

 mation of hygroscopic water in another portion of the 

 substance, the remainder will be the organic matter, or 

 other volatile matter besides water. 



c. A part of the . carbon sometimes obstinately resists 



