REAGENTS 
33 
quickly in strong alcohol, they will usually sink. If rather large 
air bubbles prevent the material from sinking, as in case of peri- 
chaetical leaves of some mosses and involucral leaves of liverworts, a 
little dissection or a careful snip with the scissors will obviate the 
difficulty. If an air-pump is available some bubbles are easily 
removed, but air bubbles in cells may resist even the air-pump. 
Heating followed by rapid cooling is recommended by Pfeiffer and 
Wellheim for removing air, but, for cytological work, the remedy is 
worse than the bubbles. 
It is often asked whether fixing agents really preserve the actual 
structure of cell contents. It must be admitted that some things— 
notably the liquid albuminoids—are much modified in appearance, 
but the most competent observers are now inclined to believe that 
such delicate objects as chromosomes, centrosomes, the achromatic 
figure, and even the structure of protoplasm, can be studied with 
confidence from material which has been fixed, imbedded, and 
stained. Extensive investigations upon various objects in the living 
condition have strengthened this confidence. 
It is certain that we have not yet found the ideal fixing agent 
for cell contents. Such an agent must not be a solvent of any of the 
cell contents, must penetrate rapidly, must preserve structures 
perfectly, and must harden so thoroughly that every detail shall 
remain unchanged during the subsequent processes of dehydrating, 
clearing, imbedding, sectioning, and staining. 
DEHYDRATING AGENTS 
Objects which are to be imbedded in paraffin or celloidin, and 
also all other objects which are to be mounted in balsam or Venetian 
turpentine must be dehydrated, i.e., they must be freed from water. 
The slightest trace of water is ruinous. Alcohol is used almost 
exclusively for dehydrating. The process must be gradual. If 
material has been fixed in an aqueous solution, it must pass through 
a series of alcohols of increasing strength, beginning with about 
3 per cent alcohol. Ten years ago, most botanists were beginning with 
35 per cent alcohol; in the second edition of this book (1905) we 
recommended 15, 35, 50, 70, 85, 95, and 100 per cent as a safe series, 
since it causes no obvious plasmolysis of the cell contents. As in¬ 
vestigations have become more and more critical, especially investi¬ 
gations upon the structure of chromatin, it has been found that even 
