32 FIXATION 



sol and the more vigorous the fixative, the greater will be the tend- 

 ency of the coagulated particles to cohere in a single clot. 



Some fixatives act in quite a different way. Instead of coagulat- 

 ing, they make protein sols more viscous or convert them into 

 gels ; if the protein is already a gel, they stiffen and stabilize it. In 

 these processes there is no dissociation of protein from water and 

 therefore no production of surfaces that will scatter light. Instead 

 of a coagulum there is a viscous sol or an aqueous gel, transparent 

 unless opaque matter has been deposited from the fixative itself. 



To observe the effects of fixatives on proteins, it is convenient 

 to begin with naked-eye observations on what happens when they 

 are added to globular proteins in the form of aqueous sols. Experi- 

 ments of this kind were made more than half a century ago by 

 Fischer ^^^ and Mann.^^^ A convenient way of carrying out such 

 experiments is described in the Appendix (p. 315). The protein 

 used is egg-albumin. 



For the benefit of those who may read this chapter without 

 having read the first, it is necessary to say once more that in all the 

 experiments with fixatives described in this book, the substances 

 were used at the concentrations shown on p. 24, except where the 

 contrary is distinctly stated. 



When a coagulant fixative is added to an albumin sol, a coherent 

 coagulum usually appears at once and occupies most of the space 

 filled by the fluid; in some cases (picric acid, mercuric chloride, 

 chromium trioxide) it has fallen somewhat towards the bottom of 

 the tube by the next day. It is either white (ethanol, acetone, tri- 

 chloracetic acid, mercuric chloride) or coloured (picric and chloro- 

 platinic acids, yellow; chromium trioxide, orange). With methanol 

 and nitric acid it is at first in the form of a fine, white flocculus ; the 

 particles eventually cohere, and indeed nitric acid produces in the 

 end rather a firm clot. With hydrochloric acid there is no immedi- 

 ate flocculation, but fine, white particles have begun to form within 

 5 minutes, and they fill the space occupied by the fluid within an 

 hour; the next day they are coherent and the tube can be held 

 horizontally without loss of water. 



When formaldehyde, osmium tetroxide, potassium dichromate, 

 or acetic acid is added to the albumin sol, no coagulum is formed. 

 (Formaldehyde solution usually contains methanol, and very 

 sparse, fine particles are then formed in the fluid.) 



It must not be thought that non-coagulant fixatives are neces- 

 sarily without effect on albumin, simply because they produce no 



