§ 90. DESICCATION. 147 



DESICCATION. 



90. One of the more frequent determinations in agri- 

 cultural analysis is that of water, ash, and organic matter. 



In the elimination of water, or the desiccation of the 

 substance or solution, the object may be to determine the 

 hygroscopic water of the substance, or it may be the 

 estimation of the total amount of solid matter in a solu- 

 tion. 



a. For the estimation of hygroscopic moisture^ dry the 

 substance well in the air, so that it shall be thoroughly 

 air-dried^ or under a bell-jar over sulphuric acid, as may 

 be directed in each special case ; then heat a weighed 

 quantity of it in a watch-glass in the steam or air-bath to 

 the temperature indicated in each case, as long as it loses 

 weight ; while weighing the substance, it should be en- 

 closed between two watch-glasses that fit well together 

 by their ground edges. 



h. Sometimes, as in the case of gypsum containing no 

 volatile matters but water, the substance can be ignited 

 at once in a covered crucible. A gentle heat should be 

 applied at first, and the temperature should be gradually 

 raised, at least almost to a red heat in some cases. 



c. When the substance contains a large amount of wa- 

 ter, as in the case of the green parts of plants, it is best 

 to dry a large, weighed quantity in a drying-chamber, at 

 from 60° to 80° C, determine the loss of weight at this 

 temperature, and then proceed as in «, with from 3 to 6 

 grms. of this partly dried substance. 



d. Sometimes the substance to be dried contains other 

 volatile ingredients, as ammonia for example ; in this 

 case the desiccation must be performed in a current of 

 dry air, or an inactive gas, like hydrogen, by means of 

 which the volatile products are carried into some absorb- 

 ing solution. Procure a deep water-batn, through which 

 a tube of the same material passes laterally, and projects 



