REACTIONS OF FIXATIVES WITH PROTEINS. I 37 



the water at |, i, 2, or 4%, the swelUng is almost exactly the same 

 as in distilled water. 



Different fixatives penetrate into protein gels at different rates. 

 Our understanding of the subject dates from Medawar's investiga- 

 tion ^^^ of the rate of their penetration into coagulated blood 

 plasma. In his experiments the fresh blood of cocks was cleared of 

 corpuscles by centrifuging and poured into tubes closed at one 

 end; the plasma in the tubes was then coagulated by the addition 

 of a trace of tissue-extract. The tube was placed in a large volume 

 of fixative. The rate of penetration of coagulant fixatives was easily 

 noted, since the plasma became opaque on coagulation. A sharp 

 line divided uncoagulated from coagulated plasma. To test non- 

 coagulant fixatives, Medawar dissolved indicators in the plasma; 

 these showed the progress of the fixative into the plasma by change 

 of colour. 



The experiments showed that in penetrating into a protein 

 gel, fixatives obey the laws of diffusion. If ^is the distance pene- 

 trated, t is time, and K a constant depending on the fixative used, 

 then 



d = K./t. 



It is convenient to measure d in millimetres and t in hours. It 

 follows that K represents the distance in mm penetrated in one 

 hour. 



When indicators are used to measure the penetration of fixatives, 

 what is being measured is the progress of the fixative at the con- 

 centration necessary to give a visible colour-reaction with the 

 particular indicator used. There is no reason to suppose that this 

 will be the same as the concentration necessary to fix the protein. 

 The following experiments were designed to overcome this 

 difficulty. ^^ For full practical details, see Appendix (p. 318). 



Gelatine/albumin was chosen as the protein gel into which the 

 fixatives were to penetrate. The gelatine gave the necessary 

 solidity, while the albumin marked the progress of coagulant 

 fixatives by becoming opaque. Small tubes were filled with the gel. 

 A rubber pipette-bulb closed one end, while the other was left 

 open (fig. 2). The tube was placed in a large volume of fixative, in 

 which it floated vertically. The penetration of coagulant fixatives 

 could easily be measured from time to time. A diamond-mark 

 near the top of the tube served as reference-point. The distance 



