MOUNTING AND PREPARATION OF OBJECTS. 4t 
4. Mounting Dry or in Air.—The method of dry 
mounting is well suited to such objects as crystals, which 
show well in air, and which may be dried without suf- 
fering alteration. It is fairly simple, although in this 
respect, as well as in permanency, it is excelled by the 
balsam mount. 
For all methods of mounting, thoroughly cleaned 
slides and covers form a first essential. Slides which 
have been already used and are soiled with balsam or 
other resinous substances must first be treated with xylol 
‘or turpentine. Otherwise all slides and covers should 
be put into a cleaning mixture made up by dissolving 
20 grams of potassium bichromate in 100 cc. of water 
and adding 100 cc. of sulphuric acid. From this they 
may be transferred to 50% alcohol, and in that solution 
kept until needed for use. Both slides and covers should 
always be handled by their edges, since the oils of the 
skin will infallibly soil their surfaces. 
Given the slide and the cover, there must be con- 
structed some sort of wall or cell which shall separate 
the two and prevent the specimen from being crushed. 
The cell will vary in depth with the thickness of the 
object, and, when a shallow cell will serve, it is only neces- 
sary to make a ring of cement and place a round cover 
directly upon it. A neat mount may be made by using 
a turntable such as is shown in Fig. 24, the slide being 
clamped on the movable portion and whirled about, 
while a brush dipped in shellac, marine glue, Bell’s 
cement, or some similar material is held just over one of 
the guide rings on the turntable with tip lightly touch- 
