no 



FIXATION 



or ethanol. The reaction with formaldehyde is very quick. The 

 reaction-products in the different mixtures containing chromium 

 trioxide and formaldehyde have not been identified. In Sanfelice's 

 fluid/^^ which contains these substances and acetic acid, the colour 

 of the reaction-products is brown; in Allen's *B.i5',^ which con- 

 tains picric acid and urea in addition, it is green. The latter colour, 

 combined with solubility, is suggestive of reduction to the ter- 

 positive ion in one of its hydrated forms. 



Chromium trioxide is compatible with all the primary fix- 

 atives mentioned in this chapter and the next, other than 

 formaldehyde and ethanol, but when it is mixed with potassium 

 dichromate, the latter substance does not exhibit its own char- 

 acter (see p. 128). 



Unclassified remarks. Bright light is destructive of cells fixed in 

 solutions of chromium trioxide. ^^ If the light of a dark-ground 

 condenser is focused on a cell while it still lies in the fixative, the 

 following changes may be observed. The outline of the cell be- 

 comes hazy; the particles of coagulated cytoplasm begin to show 

 Brownian movement and soon disintegrate, leaving only the lipid 

 globules and nucleus; the latter then dissolves, the nucleolus re- 

 sisting solution longer than the rest of it; finally nothing is left 

 except the lipid globules. *^^ The result is the same when heat- rays 

 from the microscope-lamp are filtered off. 



The action of chromium trioxide as a fixative will be referred to 

 again in the section on potassium dichromate (p. 128). 



