472 BOTANICAL GAZETTE [DECEMBER 
The reservoir should be about half full in order to have as 
much surface of mercury exposed to the action of the acid as 
possible, and should be kept always stoppered. If mercurous 
sulfate is not at hand, it can be made by gently heating mercury 
with concentrated sulfuric acid (1° mercury to 10° acid), being 
careful that there is always an excess of acid. This should be 
done in a hood. 
The reservoir may be placed against the wall by using a ring 
supported by staples, as shown in the cut. 
PURDUE UNIVERSITY, 
LAFAYETTE, IND. 
