The specific oxygen capacity of blood 



by water, is very great. We must however provisionally conclude that 

 haemoglobin differs in different species, and perhaps even in different 

 individuals. This difference may be explained, in part at all events, 

 by a difference in the globin portion of the molecule, which, on account 

 of its relatively great weight, plays a predominant part in the data of 

 chemical analysis, whilst it is but by-play in the study of haemoglobin 

 as a respiratory pigment. It would be enough for our purpose could 

 we show some constancy in the iron-containing portion of the molecule 

 and leave on one side the question of the exact composition of the 

 protein with which it is combined. 



For this reason much laborious work has been done on the deter- 

 mination of the " specific oxygen capacity " of haemoglobin, that is, 

 on the number of cubic centimetres of loosely combined oxygen 

 which correspond to every gram of iron in the compound. Suppose 

 for instance that it appears, as the result of analysis, that 401 cubic 

 centimetres of oxygen correspond invariably to a gram of iron. It 

 would follow that haemoglobin obeyed the law of definite proportions 

 so far as oxygen and iron were concerned, and also that it obeyed 

 the law of combination in simple proportions. For, expressing this 

 ratio of the iron to the oxygen by weight, every 56 grams of iron 

 would correspond to 32 grams of oxygen. In other words, the 

 oxygen and the iron would be united in the proportion of one atom 

 of iron to two of oxygen. 



The following table will however show that the history of the 

 subject provides us with but scanty hope of reaching this ideal (3) . 



The discrepancies between the various analyses amount in some 

 cases to one-third of the whole quantity of oxygen measured, and 

 some little consideration must be given to their interpretation. 



12 



