123 



107. Mounting in balsam. Certain preparations may be 

 mounted in balsam by drying, the method of desiccation ( 100), e. 

 g., cover- glass preparations of bacteria, stained cover-glass prepara- 

 tions of blood, etc. For this it is only necessary that the prepara- 

 tion be absolutely dry, a small drop of balsam placed upon it or 

 upon the under side of the cover- glass, which is carefully placed 

 over the specimen and pressed down. 



Mounting in balsam by desiccation is serviceable for but few 

 preparations in histology, and in most cases the removal of the 

 water by a series of displacements is resorted to ( 100). For this 

 the following steps are necessary : Dehydration, Clearing, Mount- 

 ing in balsam. 



Dehydration. The sections are entirely freed from water by 

 the use of 95% or absolute alcohol. The slide or free section may 

 either be placed in a jar of alcohol or alcohol from a pipette be 

 poured over it. Treat the preparation to be mounted for 5-15 min- 

 utes. The thicker the section the longer the time required ; collo- 

 dion sections require a longer time than paraffin sections. In any 

 case, be sure that the dehydration is complete, giving a longer 

 rather than a shorter time, and then clear. 



108. Clearing. This is accomplished by putting the slide 

 in a jar of clearer or dropping the clearer upon the section from a 

 pipette. When the section is cleared it will be transparent. Test 

 it by holding it against a dark background ; if it is not cleared it 

 will be* cloudy, white, and opaque. 



109. Mounting in balsam. Drain off the clearer and allow 

 the section to stand until there appears the first sign of dullness 

 from evaporation of the clearer from the surface. Then place a small 

 drop of balsam upon the section or upon the cover-glass which is 

 then inverted over the specimen. 



Remember that in mounting in this way you must always ' ' De- 

 hydrate, Clear and Mount in Balsam" and that the three steps 

 are inseparable. 



SEALING THE PREPARATIONS. 



This is only necessary when the preparation is a glycerin or 

 glycerin-jelly mount. It is better not to seal balsam preparations, 

 or only quite late after the balsam has thoroughly dried out. 



