The acquisition of oxygen by the blood in the lung 209 



readings which are consistent with those given by the blood gas 

 pump. This comparison proved very illuminating. It was of course 

 necessary to calibrate the instrument in some way ; there was no 

 antecedent probability that the movement of the band would be a 

 direct linear function of the percentage of CO-haemoglobin present. 



100 



50 



o so 100 



FIG. 102. The calibration curve. Ordinate = percentage of haemoglobin which is 

 CO haemoglobin. Abscissa =r micrometer readings of instrument. 



Hartridge began by calibrating the instrument with mixtures of CO- 

 and oxyhaemoglobin in known amounts or amounts at least presumed 

 to be known i.e. it was presumed that the amounts bore the same 

 relation in the mixture that they did in the aliquot parts. So long as 

 Hartridge continued to use this method his comparisons with the 



B. R. P. 14 



