160 CHEMICAL STATICS 



sequently unites with the oxygen. Barcroft and Roberts (9) (10) 

 (11) (12) (13) have, however, shown that the irregularities ob- 

 tained by Bohr were due to the presence of electrolytes in the 

 haemoglobin solutions; in dialysed solutions the curve of dis- 

 sociation is exactly that demanded by Hiifner's theory. The 

 velocity of dissociation of oxyhaemoglobin obeys the equation 

 indicated by the mass law. The variations of the equilibrium- 

 constant (K) with change of absolute temperature follow the 



van't Hoff equation 



1 dK ^ -q 



K' dt ~ 2T^' 



where q is constant and equal to 28,000 calories. This is there- 

 fore the heat of combination of one gram-molecule of hsemoglobin 

 with oxygen. Since the amount of heat which is actually given 

 out when one gram of hsemoglobin unites with oxygen is 1.85 

 calories, the molecular weight of hsemoglobin, in dialysed solu- 

 tion is, according to these results, 15,000. The minimum weight 

 indicated by Hiifner's results, cited above, is 16,669. A similar 

 figure is indicated by the iron-content of the hsemoglobin mole- 

 cule (55) (121) (49) (50) and by the osmotic pressure measure- 

 ments of Hiifner and Gausser (51). Weymouth Reid (98), 

 however, found a value, as indicated by osmotic pressure meas- 

 urements, three times as great. Barcroft and Hill (11) suggest 

 that hsemoglobin may possibly exist as a polymer of itself, under 

 certain conditions, and that the irregularities observed in the 

 curve of dissociation of oxyhsemoglobin in the presence of elec- 

 trolytes may be due to the breaking down of such aggregates. 

 The temperature-coefficient of the dissociation of oxyhsemoglobin 

 is large, about 4 per 10 degrees rise in temperature. 



These masterly investigations of Barcroft effectually demon- 

 strate two things; in the first place that the union between 

 hsemoglobin and oxygen is not an "adsorption combination" 

 as Wo. Ostwald has suggested (90) (101) (13) and, in the second 

 place that, since the derivation of the van't Hoff law (reaction 

 isochore) involves the assumption that the gas laws apply strictly 

 to the system under consideration,* hsemoglobin, although a 

 colloid, does not form a separate phase within its solution,! but 



* Cf. W. Nernst (83). 



t It may be remarked that under such conditions haemoglobin cannot 

 present any surface with which to "adsorb." 



