MOUNTING AND PREPARATION OF OBJECTS. 41 



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 sUde being 

 damped 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- 



