142 HEMOGLOBIN 



the partial pressures of, and are therefore measures of, the oxygen 

 present physically dissolved in solution I. It is clear that the greater 

 the quantity of oxygen which is thus dissolved, the longer will the 

 hydrosulphite take in ridding the mixture completely of it, and there- 

 fore the longer the time which will elapse before the reduction of the 

 haemoglobin commences. Herein Ues the significance of the fact that 

 the graph marked 7 mm. O2 descends abruptly from the origin: any 

 decrease in the dissolved oxygen below 7 mm. entails an appreciable 

 reduction of the haemoglobin. The two remaining graphs descend 

 but Uttle from the 100 per cent, level for a time — longer in the case of 

 the solution which has the higher concentration of free oxygen. It is to 

 be noted that the three graphs, once they do take a definitely down- 

 ward course, are parallel to one another, showing that the reaction 



HbOa -^ Hb + O2 



proceeds at the same rate in each case. 



As has been indicated the curves gradually tend to become less 

 steep, because the amount of oxyhaemoglobin broken up in a given 

 moment of time depends upon the amount of oxyhaemoglobin which 

 is left unbroken in the solution at that time. If, however, this law 

 is rigidly fulfilled — and it is simply the law of mass action adapted 

 to this particular case — the logarithm of the concentration of oxy- 

 haemoglobin, when plotted against the time through which the reaction 

 has run, should give a straight line. This is another way of saying 

 that the velocity constant of the reaction is the same from start to 

 finish, and of saying that if the same concentration of oxyhaemo- 

 globin were present in each case, the time necessary for a certain 

 smaU proportion of it to be broken up would be the same irrespective 

 of the amount of reduced haemoglobin with which it were mixed, 

 i.e. irrespective of the percentage saturation. 



Fig. 43 shows that Hartridge and Roughton's determinations, 

 when plotted in the way indicated above, fall on a straight Hne with 

 exactitude of a remarkable order considering the difficulties under 

 which they were made. 



On page 139 an experiment was cited in which the velocity constant 

 for the reaction 



Hb -f- O2 — ^ HbOa 



was given as being represented by the figure 132. A determination 

 of the velocity constant of the opposite phase, viz. 



HbOa— ► Hb-H O2, 



