692 PHARMACEUTICAL BOTANY 
while hot through glass-wool or filter paper and collect the 
filtrate in a wide mouthed bottle. Keep well stoppered so as to 
exclude dust. 
Glycerin-gelatin becomes solid when cool. For use warm the 
bottle in a water bath after first removing the stopper. A glass 
rod sufficiently long to reach to the bottom of the bottle can be 
inserted in the cork and used for transferring the material to the 
slide. 
8. Ring mount with zinc white, balsam or asphaltum at the 
edge of cover slip. If the cover slip is circular, this can best be 
done by means of a centering turn-table. A camel’s hair brush is 
dipped into the zinc white, balsam or asphaltum and held to the 
margin of the cover slip while the slide, fastened with clips to the 
turn table, is rotated with it. 
9. Label slide. 
If the objects or sections are such as not to be liable to shrink they can 
be transferred from water or weak glycerin directly to glycerin-gelatin. 
TECHNIQUE OF Fixinc, DEHyDRATING, HARDENING AND 
IMBEDDING IN PARAFFIN 
When the intention is to study the protoplasts in their natural * 
form or the processes of cell division, the fresh material must be 
put through the various stages of fixation, hardening and imbed- 
ding before it is sectioned. The steps will now be considered in 
the order in which they must be carried out. 
Fixation.—This is the process of killing and coagulating the 
protoplast. The essence of good fixation is in rapid killing. It 
should be simultaneous with coagulation or hardening so that the 
protoplast will not be modified by later treatment. Fixing 
fluids are always substances unknown to protoplasm, ¢.g. 
poisons. The coagulation of protoplasmic structures is due to 
the fact that these are alkaline in reaction whereas the fixing fluid 
is acid. Fixing fluids must be judged not only as to killing and 
hardening but also as to reaction of tissues to stains afterward. 
Fluids that are mixtures make the best fixing agents. Among the ° 
fixing agents employed are the following: Osmic acid (OsO,) 
comes in sealed glass tubes containing 0.5 gm. or 1 gm. It has a 
very powerful odor and is easily affected by organic materials. 
