Tributes 



delight when we found that point after point on the curve for human 

 blood lay also on the curve of the haemoglobin solution.' Later work 

 of Barcroft and Peters showed that variations of hydrogen ion con- 

 centration had much greater effects on the position of the curve than 

 variations of salt content, and in the second edition of his book 

 Barcroft said : ' I never know whether it was more fortunate or un- 

 fortunate that the observation [i.e. the effect of salts] was made at so 

 early a stage. It was unfortunate, because at that time the effect of 

 hydrogen ion concentration was not appreciated and therefore not 

 controlled ; moreover, the distilled water solution itself was not so 

 free from salts as it would have been had the investigation been made 

 a year later. The fortunate circumstance, however, was that had the 

 discovery not been made at the time it was, it might have been 

 long postponed, for the whole effect of salts on the dissociation 

 curve might have been attributed to changes in the hydrogen ion 

 concentration.' 



This effect of salts suggested to Barcroft and Roberts that an inves- 

 tigation should be made of the dissociation curve of haemoglobin, 

 which had been dialyzed against distilled water. The results were 

 remarkable. The curve changed from a sigmoid type to one more like 

 the rectangular hyperbola predicted by the Law of Mass Action, 

 assuming that the haemoglobin molecule contains one atom of iron. 

 Although this effect has now been known for almost forty years, and 

 accounts of investigations of it are still being published, a complete 

 explanation is not yet available. The first and most brilliant suggestion 

 was due to A. V. Hill who was then working with Barcroft. He 

 suggested that when dissolved in pure water, haemoglobin might exist 

 in single molecules and in the presence of salts it might aggregate to 

 form a polymer with the average number n = 2*5. Before 1914 this 

 aggregation theory was tested in many ways by Barcroft and his col- 

 leagues and the results obtained appeared to agree with the require- 

 ments of the theory. After the First World War, when I had the 

 privilege of working with Barcroft, we planned to determine the value 

 of n by independent methods on the same haemoglobin solution, 

 namely by determinations of oxygen dissociation curves and measure- 

 ments of osmotic pressure. As in so many investigations, early results 

 seemed most encouraging, but more detailed studies did not confirm 

 the theory. In dilute solutions a value of n = 4 was obtained by osmotic 

 pressure measurements. About the same time Barcroft planned a 

 systematic investigation of oxygen and carbon dioxide contents of the 

 blood of a series of normal human subjects and a comparative study 

 of the haemoglobins of the same subjects. In this work the most 

 highly purified haemoglobins did not give hyperbolic curves. 



25 



