CH. II.] HEAT COAGULATION. 43 



a considerable part of this is aggregated into flocks (see 

 p. 11). At high temperatures these flocks are coagulated, 

 that is, they do not redissolve on altering the reaction of the 

 fluid. 



The main effect of neutral salts is to aid the aggrega- 

 tion of the denaturised protein. It often happens that 

 more than one denaturised protein is formed by heating a 

 solution containing albumins and globulins. The iso- 

 electric points of these proteins may differ, so that a certain 

 proportion of the protein will remain non-coagulated at any 

 given reaction. This seems to be true, even in the case of 

 the denaturised protein formed from a pure protein. The 

 addition of a neutral salt will tend to cause flocking of this 

 " soluble " portion of the denaturised protein, and these 

 flocks will be coagulated should the temperature be high 

 enough. The non-coagulated dispersed protein carries an 

 electric charge, which varies with the reaction, being posi- 

 tive in acid solutions and negative in alkaline solutions. 

 We have seen that electro-positive colloids are flocked by 

 negative ions, and electro-negative colloids by positive 

 ions. Further, that this flocking power of the ions is much 

 greater with di- and tri-valent ions than with mono-valent 

 ions. It follows, therefore, that the ideal conditions for the 

 maximal heat coagulation of an albumin or globulin are 

 that the reaction should be at the iso-electric point of the 

 denaturised protein formed, and that there should be 

 present di- or tri-valent ions both positive and negative. 

 These conditions are met by having the boiling solution 

 very faintly acid to litmus and adding a trace of calcium 

 chloride or magnesium sulphate. It is advisable to add the 

 salt, to boil the solution, and then to change the reaction 

 slowly by the addition of dilute sodium carbonate or i 

 per cent, acetic acid, so that the final reaction is just acid 

 to litmus. The exact point and procedure can only be 

 determined by experience since it varies considerably with 

 the concentration of the protein, etc. 



