RESPIRA TION 



621 



Barcroft and Hill (see Barcroft's book, 1914, Chapter III.) made experiments to 

 determine the heat evolution, and found a value of 1/85 calories per gram of 

 haemoglobin. From the formula of van't Hoff, it is possible to calculate the 

 molecular weight of haemoglobin, on the assumption that each gram combines with 

 1-34 c.c. of oxygen. The result came out nearly identical with the accepted 

 molecular weight, 16,669, and it is clear that it affords considerable support to the 

 view of true chemical combination. But here we come across another puzzle. 



FIG. 191. 



20 30 40 50 60 70 80 90 100 



OF ELECTROLYTES ON THE DISSOCIATION CURVE OF HEMOGLOBIN. 



Ordinates percentage saturation of haemoglobin with oxygen. 

 Abscissae tension of oxygen in mm. mercury. 

 curve from dialysed solution. 

 ] curve from undialysed solution. 



The first curve (electrolytes absent) corresponds to HUfner's curve and is a rectangular hyperbola. It 

 passes very nearly through the experimental values. 



The second curve (salts present, in low concentration) is Bohr's curve. 



The difference between the degree of saturation is especially marked at the lower oxygen tension. 



(Barcroft and Roberts, Jl, Physio/., 39, 146.) 



The heat of combination of oxygen and haemoglobin has been determined by other 

 experimenters, and results considerably lower than that mentioned have been 

 obtained; the numbers may be found in Meyerhof's paper (1912, 1, p. 164). If 

 we consider only that of Torup (1906), which was obtained by a method 

 essentially the same as that of Barcroft and Hill, and there is no apparent reason 

 to doubt the accuracy of the determination, we find only 0-678 calorie per gram. 

 R. du Bois-Reymond (1914) found values between 1'06 and 1*77, in the 

 mean, 1'36. 



In the consideration of the problem we must not forget that the condensation of gases on 



