120 HAEMOGLOBIN 



even in it the oxygen would appear to have a definite relation to the 

 iron atom. 



The whole subject has been put very well in a letter written by 

 Neil Adam(3) to Nature which, with the author's permission and that 

 of the editor of the magazine, I quote. This letter has the advantage 

 of defining adsorption in such a way as to make it something more 

 definite than is vaguely conceived in most people's minds. 



THE COMBINATION BETWEEN OXYGEN AND HEMOGLOBIN 

 AND THE CRITERIA OF ADSORPTION 



Haemoglobin combines with oxygen approximately in the ratio of 16,670 to 32, 

 by weight, as was shown by Peters {Journal of Physiology, vol. XLiv. p. 131). It is 

 clear, then, that in solution the particle of haemoglobin is very much larger than the 

 particle of oxygen which combines with it. If one might assume that the densities 

 and shapes of the particles were similar, then their surfaces would be in the ratio 

 64 to 1 ; in any case, and whatever the degree of aggregation of the particles, probably 

 only a very small part of the surface of the haemoglobin particles can be actually 

 covered by oxygen when combination ceases at the stage of oxyhaemoglobin. 



This shows that the attraction of haemoglobin for oxygen is a highly localised 

 property of the haemoglobin particles. For if this attraction were more or less evenly 

 distributed over the surface, it would be satisfied only to a small extent, when a small 

 part of the surface was covered, and at higher concentrations of oxygen than those 

 which are found experimentally to give saturation with oxygen, more oxygen would 

 be taken up. 



Taking the thermal motions of the particles into account does not aflFect this argu- 

 ment, since the movements of the particles according to the laws of the kinetic theory 

 do not affect their surface areas. 



Now, if the attraction of haemoglobin for oxygen is of such a character that it is 

 satisfied when only a small portion of the surface is covered, it seems impossible to 

 regard this combination as a case of adsorption. 



The criteria of adsorption are perhaps not yet so well defined as could be wished, 

 if differences of opinion as to whether a given process should be classified as adsorption 

 or not are to be avoided. I am inclined to think that a process is rightly classified 

 as adsorption, if the substance taken up by the surface continues to be taken up until 

 the whole surface is uniformly covered, but not otherwise. Covering the surface imiformly 

 is of course meant in the sense in which a gas or homogeneous solid is said to fill 

 space uniformly; that is, \miformly to a being armed with a microscope to which 

 individual atoms are small. 



This definition is both definite theoretically, and in accordance with common con- 

 ceptions of adsorption. It is difficult, indeed, to see what other definition is possible 

 in the present state of knowledge. It is perhaps, however, desirable to state the 

 definition clearly; although, as I feel it must have been present, whether formulated 

 or accepted as self-evident, to the minds of many workers on adsorption, no sort of 

 novelty is claimed for it here. 



A definition of adsorption based on the nature or quality of the forcee attracting 



