The dissociation curve of haemoglobin 25 



apparatus, and the amount which it would afterwards give out when 

 treated with potassium ferricyanide. When due allowance had been 

 made for the fact that "pumped-out blood" absorbs the carbonic- 

 acid of the air as well as the oxygen, and for other trifling physical 

 factors, Burn obtained the following figures : 



Each figure represents the average of four determinations: clearly 

 dilution has no effect on the oxygen capacity. 



In passing it is interesting to note that Burn was able to simulate 

 some of Manchot's results, when he used only the absorption method. 



A caution must here be given against the attractiveness of 

 analogies between certain inorganic oxidations and that of haemo- 

 globin. There is perhaps no greater danger at the present time, to 

 one who would research in this particular subject, than the temptation 

 of trusting to analogies instead of to analysis. 



Before leaving the point of analogies we must warn the reader 

 against one which is frequently put forward, namely, that of the 

 union between calcium oxide and carbonic acid. In this case if the 

 pressure and the amount of carbonic acid united to the calcium 

 oxide be plotted after the manner described above we get quite a 

 different type of curve. At a certain critical pressure the carbonic 

 acid begins to come off" and at that pressure it all comes off. The 

 essential difference between the two cases lies in the fact that, in the 

 case of the haemoglobin reaction the reacting substances are all in 

 solution, while in the case of the calcium carbonate the reaction is 

 between a solid and a gas and therefore involves a change of phase. 

 The number of phases being one greater, the number of degrees of 

 freedom is one less. 



