REACTIONS OF FIXATIVES WITH PROTEINS. I 33 



visible result in this experiment. It can easily be shown, for in- 

 stance, that both formaldehvde and osmium tetroxide can render 

 albumin non-coagulable by ethanol ^^ (see Appendix, p. 320). It 

 has been known for many years that osmium tetroxide can set 

 undiluted egg-white into a gel, without coagulation. ^^ The ability 

 of osmium tetroxide to make protein sols into gels naturally 

 depends on the concentration of the protein. Thus 12% serum 

 albumin is gelled but 8% is not.^^^ Different proteins also differ 

 considerably, serum globulin being more easily gelled than 

 albumin, and fibrinogen than globulin. ^"^^ Such gels are generally 

 opaque. They have a tendency to liquefy eventually. '*^^' ^^ 



Although potassium dichromate does not coagulate albumin 

 sols, it does gradually render undiluted egg-white more viscous 

 and eventually transforms it into a weak, semi-transparent gel. 

 (See Seki.*^^ Potassium dichromate was used in the form of 

 Miiller's fluid, ^^^' ^^^ that is, with the addition of sodium sulphate.) 

 Acetic acid has no such effect. 



There is unfortunately no substance that will represent the 

 fibrous proteins of the tissues so conveniently in test-tube experi- 

 ments as albumin will represent the globular ones. Gelatine gel 

 (25% w/W) can be used. It is convenient to cast the material in 

 15- or 30-grain pessary moulds. IVIost fixatives will not stabilize 

 these gels in such a way that they retain their form when put in 

 warm water. Gels that have been left for 24 hours in a solution of 

 acetic acid or potassium dichromate dissolve in water at 37° C in 

 a quarter of an hour or a little more. After fixation in methanol, 

 ethanol, picric acid, mercuric chloride, or chromium trioxide, the 

 gels resist complete solution for an hour or more but eventually 

 dissolve. Formaldehyde and osmium tetroxide contrast strongly 

 with these fixatives, for the gel does not dissolve, even if kept in 

 water at 37" C for days. The gel fixed by osmium tetroxide is 

 black and swells slightly in the warm water; that fixed by formalde- 

 hyde retains its original appearance, and one would not guess that 

 it had been fixed. 



Interesting information can be obtained from experiments with 

 gelatine gels containing an admixture of albumin. Instructions for 

 preparing gelatine/albumin gel are given in the Appendix (p. 314). 

 The gel is cast in pessary moulds. It will serve for certain purposes 

 as a crude model of protoplasm. The refractive index is about 

 1*365,^^^ which is within the range shown by the protoplasm 

 of ordinary cells. Thus the proteins are at about the same 

 c 



