QO FIXATION 



non-coagulant 



formaldehyde . . . Chapter 6, p. 1 1 1 



osmium tetroxide . . . ,, p. ii8 



potassium dichromate . . ,, p. 126 



acetic acid .... ,, p. 134 



These fixatives will be considered one by one under a standard- 

 ized set of headings. The headings are listed below, with short 

 comments w^here necessary. 



Standard concentration for fixation. Most of the simple fixatives 

 can be used over a fairly wide range of concentrations, but it is a 

 great convenience to select a rather arbitrary 'standard' concentra- 

 tion, somewhere within this range. It is better to know something 

 about 5% acetic acid, for instance, from all points of view, than to 

 have disjointed information about its pH at i %, its rate of penetra- 

 tion at 2'5%, and its swelling effect at 10%. The standardization 

 of concentrations also makes it possible to avoid unnecessary 

 repetition. Wherever in this book a primary fixative is mentioned, 

 it is to be understood, unless the contrary is distinctly stated, 

 that the information refers to its use at the standard concentration, 

 or at a concentration so close to this that the difference is 

 insignificant (see pp. 24 and 32). 



Description. Information about the appearance of each sub- 

 stance and its melting- and/or boiling-point, solubility, etc., is 

 collected under this heading. Industrial uses are also mentioned. 



Ionization. The figures given in the literature for the pH of 

 aqueous solutions at the concentrations suitable for fixation are 

 not always in good accord. This must be attributed partly to the 

 impurity of the chemicals commonly used in biological labora- 

 tories. Two of the fixatives are used at saturation, and temperature 

 may have a considerable effect on the concentration of such 

 solutions and hence on pH; it may also affect the pH of other 

 fluids. 



The data on change of pH during fixation are those of Freeman 

 and his collaborators ^"^ and Casselman.^^^ Freeman generally 

 allowed i g of spinal cord or liver of cat to 25 ml of fixative 

 solution. 



Oxidation-poteiitial. The oxidation-potentials of fixatives have 

 been investigated by Casselman,^^^ whose figures are quoted in 

 this chapter. 



Manufacture. For fuller information about proce$§Q§ of manu- 



