RESPIRATION 615 



Douglas, Haldane, and Haldane (1912) point out that the relative affinity of haemoglobin 

 for oxygen and carbon monoxide varies in different individuals. They regard this as due to 

 the globin, since the hsematin part is always the same. If so, it is difficult to see how the 

 iron, which is a constituent of the latter, is alone concerned with the taking up of these gases. 



Fischer and Brieger (1912) have made an interesting investigation of the behaviour of 

 certain iron compounds to oxygen. They regard the combination of oxygen in the blood as 

 an analogous case and that it is in the form of a peroxide, which is stable in alkaline solution, 

 unstable in acid solution, similar to the ferrates and ferrites which they have prepared. At 

 present, however, it is difficult to bring these results into comparison with the system of 

 haemoglobin and oxygen, since they were obtained by the use of hydrogen peroxide as source 

 of oxygen, and I cannot find evidence in their work that the relative proportion of ferrate and 

 ferrite is determined by the tension of oxygen gas. 



Let us now consider the fact which has already been incidentally referred 

 to. Let us expose haemoglobin to oxygen at a pressure of only 10 mm. of 

 mercury. We find that the amount of oxygen taken up by it is 55 per 

 cent, of that present in saturation (Barcroft, 1914, p. 16). If exposed to a 

 pressure of 40 mm. of mercury, it is 84 per cent, saturated and so on. We thus 

 obtain a curve, such as is given in the plate opposite p. 16 of Barcroft's book 

 (1914). This relationship was carefully worked out by Barcroft and Camis 

 (1909), and is known as the "dissociation curve" of oxyhsemoglobin. We shall 

 find presently that the form of the curve varies with temperature and with the 

 presence of electrolytes, but, for the present, we will merely take the fact that 

 the amount of oxygen taken up is in proportion to the pressure of oxygen, 

 that is to the concentration of oxygen present in the solution. 



Now this fact has not sufficiently aroused the astonishment of investigators. 

 Assuming that oxyhsemoglobin is a chemical compound of oxygen and haemo- 

 globin, we naturally look around for similar ones, but, so far as chemical 

 compounds are concerned, our search is in vain. There is none like it known 

 to the chemist. Certain systems have, indeed, been hastily given as analogous ; 

 let us examine them, since they are instructive in themselves. 



Dissociation of Calcium Carbonate. Calcium oxide combines with carbon 

 dioxide at ordinary temperatures to form the carbonate and, if this is heated, 

 as in the lime kiln, the carbon dioxide is again driven off and the oxide 

 obtained. It has been stated, probably from a misunderstanding of the table 

 of Le Chatelier (1883), a part of which is given below, that, at a given 

 temperature, different pressures of carbon dioxide are in equilibrium with 

 different relative proportions of the carbonate and oxide, just as there are of 

 haemoglobin and oxyhaemoglobin in equilibrium with oxygen at different pressures, 

 if we assume that oxyhsemoglobin is a chemical compound. 



TABLE OF LE CHATELIER 



Temperature. Pressure in cm. Mercury. 



547 2-7 



625 5-6 



745 ..... 28-9 



812 76-3 



865 ..... 133-3 



It is somewhat difficult to explain the meaning of the numbers in the above 

 table without using expressions derived from the phase rule, which would tend 

 to confuse the issue as regards our present problem. In the first place, we 

 must confine ourselves to one temperature, as is obvious, and assume that calcium 

 carbonate and oxyhaemoglobin are analogous ; so that, taking the first line of the 

 table, let us suppose that a temperature of 547, with calcium carbonate, 

 corresponds to one of 15 in the case of oxyhaemoglobin. This is, of course, 

 admissible. Now the table states that the dissociation pressure of calcium 

 carbonate at 547 is 2'7 cm. of mercury. That is, calcium carbonate is in 

 equilibrium with carbon dioxide gas at that pressure, so that no change takes 

 place. Next suppose that, without changing the temperature, we reduce the 

 pressure of carbon dioxide to 1 cm. of mercury, and maintain it at this level 

 by the use of a relatively large volume of gas, as we do when dealing with 

 haemoglobin and oxygen. What happens is that carbon dioxide comes off, and 



