55 



tion 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 to 15 

 minutes. The thicker the section the longer the time required; 

 celloidin 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. 



159. 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. Carbol-xylene (melted carbolic 

 acid, 1 part; xylene, 3 parts); xylene; or certain essential oils (ori- 

 ganum, thyme, cajuput, bergamot) are used. 



160. 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 \vhich 

 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. 



161. Natural balsam is acid in reaction due to organic acids contained. 

 As these bleach basic dyes, notably hematoxylin, it is well to use for most purposes 

 balsam that has been dried out and redissolved in a known solvent, such as 

 xylene or benzene. Neutral balsam solutions are to be preferred. Alkaline 

 balsam is sometimes preferable for some hematoxylin stains; acid balsam in 

 certain other cases (fuchsin acid, injection masses, 118, 217 ). Furthermore, 

 according to Mann [36] , such solvents as xylene readily oxidize with the formation 

 of acid products. For delicate work, therefore, it is probably advisable to use as 

 thick balsam as is convenient, and avoid inclusion of air bubbles in the mounting. 

 Benzene balsam from this standpoint is preferable to xylene balsam. 



Certain resinous mounting media, Camsal Balsam, Euparal, have been 

 recently prepared in which specimens may be mounted direct from 95% alcohol; 

 clearing is then unnecessary, of course. 



SEALING THE PREPARATIONS. 



162. Sealing glycerin mounted specimens. Wipe away the 

 superfluous glycerin as carefully as possible with a moist cloth or a 

 piece of lens paper. Place four minute drops of cement carefully at 

 the edge of the cover at the four quarters and allow them to harden 

 for half an hour or more; these will anchor the cover-glass and the 



