GENER.\L TECHNIC 23 



haematoxylin-staincd specimens in glycerin strongly tinged with 

 eosin (eosin-glycerin) . The specimen in a drop of glycerin is trans- 

 ferred to the glass mounting slide, the excess of glycerin removed with 

 filter paper or with a pipette, and a cover-glass put on. 



Glycerin mounts, as a general rule, are unsatisfactory. Further- 

 more, they must be cemented to exclude the air. This can be done 

 by painting a ring of gold-size, or a thick solution of gum shellac in 

 alcohol to which a little castor oil has been added, around and over 

 the edge of the cover-glass. Both cover-glass and slide must be 

 cleaned free from glycerin before the cement is applied. A camel's- 

 hair brush moistened with alcohol is the best means of removing the 

 excess of glycerin. 



Glycerin jelly is a more satisfactory mounting medium than pure 

 glycerin. It can be purchased from firms deaHng in microscopical 

 supplies and needs merely the application of heat to make it fluid. 

 Specimens can then be mounted in it in the usual manner, and after 

 being allowed to cool, do not require cementing. 



(2) Balsam. — This is the most satisfactory general mounting 

 medium. It has an advantage over glycerin in drying down perfectly 

 hard and thus needing no cement, and in preserving colors more 

 permanently. Its disadvantage is that its refractive index is so 

 high that it sometimes obscures the finer details of structure, espe- 

 cially of unstained or slightly stained specimens. 



Specially prepared Canada balsam is dissolved either in xylol 

 or in oil of cedar, the solution being made of any desired consistence. 

 Xylol-balsam dries much more quickly than does the oil-of-cedar 

 balsam. 



Preparatory to mounting in balsam, stained sections must be 

 thoroughly dehydrated and then passed through some medium 

 which is miscible with both alcohol and balsam. This medium, 

 which at the same time renders the section transparent, is known 

 as a clearing medium. For celloidin specimens the most satisfactory 

 are: 



(i) Oil of origanum Cretici. 



(2) Carbol-xylol (xylol, 100 c.c, carbolic-acid crystals, 22 gm.), 

 followed by pure xylol. 



(3) Xylol and cajeput oil, equal parts, followed by pure xylol. 



After clearing, the section is transferred by means of a section- 

 lifter to a glass mounting slide. In case oil of origanum is used, it 

 is then blotted firmly with filter paper to remove the excess of oil. 



