6] 



Desiccators 



5 



by either lumps of calcium chloride or pieces of pumice saturated 

 with strong sulphuric acid. The latter is objectionable from the 

 fact that in carrying the desiccator about the acid is apt to be 

 splashed up on to the substances above it. A very convenient 

 method of charging the desiccator is to put a couple of 

 handfuls of dry washed sand 

 in the bottom compartment, 

 and pour just sufficient strong 

 sulphuric acid on to the sand 

 to form a firm mass which 

 will not splash about. 



A large desiccator cap- 

 able of holding larger vessels 

 is made by placing a bell jar H 

 on a greased ground-glass 

 plate. Inside the chamber 

 thus formed is placed a glass dish containing some drying 

 agent and covered with a piece of perforated zinc. 



If a desiccator be quite air-tight, the air which has been 

 heated by the introduction of a hot vessel will contract as it 

 cools. When such a desiccator is opened the draught caused 

 by the inrush of air will often displace light substances from 

 the vessel in which they are contained, and thus spoil analyses 

 at the last moment. This may be avoided by making a deep 

 file-mark on the rim of the vessel, which, when the lid is 

 replaced, will leave a fine communication between the outer 

 and inner air. This will not render the desiccator any less 

 reliable. 



Fig. 4. — Desiccator. 



DRYING OF SOLIDS 



6. The water contained in many solid bodies is of two 

 kinds, viz. : — Adherent water or moisture, and combined water 

 or water of crystallisation. Moisture may always be removed 



