28 



lighter than really is the case. Again, many substances 

 can not be exposed to the air without absorbing atmos- 

 pheric moisture and in this way introducing an error into 

 the weighing. For these reasons it is customary always to 

 cool the dishes in a device known as a desiccator (fig. 25) 

 before weighing them. A desiccator is a specially con- 

 structed covered jar containing a substance like calcium 

 chlorid, which attracts to itself all the atmospheric mois- 

 ture in the inclosed space surrounding it. The desiccator, 

 containing no moisture, will, of course, permit a substance 



FIG. 24. Analytical balance. 



to be kept in it without absorbing any. The calcium 

 chlorid, which forms a layer about 1 inch deep on the bot- 

 tom of the desiccator, should be renewed as soon as it shows 

 any signs of moisture. The cover of the desiccator should 

 be removed only as often as is necessary, and then for the 

 shortest possible time. 



Milk dishes. These are commonly made of aluminum 

 and should be from 2 to 2} inches wide and about one-half 

 inch deep (fig. 26). Each dish should bear a number by 

 which it can be identified; this number may be scratched 

 or punched on the side. 



