2 



Chapter I 



Consider the question from the standpoint of evolution. If the 

 wing or the beak of a bird is too short for the highest efficiency it 

 can be modified in successive generations by a process of natural 

 selection until it attains the most suitable proportions. But in dealing 

 with chemical substances nature is set a very different task : either 

 they occur as stable compounds or they do not occur. Unless we are 

 to suppose that in times gone past there has been an infinite variety 

 of possible and impossible chemical combinations, we must suppose 

 that nature so far from adapting chemical substances to herself has 

 had to adapt herself to the chemical substances which exist. And 

 therefore I will adopt as a basis for the consideration of respiration 

 an investigation into the chemical nature of the substance haemo- 

 globin on which respiration, in the higher animals at least, depends. 

 I will begin therefore by considering the hard facts with which we 

 have to do in the chemistry and physics of the subject. 



The first question which I would answer is, Is haemoglobin a 

 single substance ? In books on Chemistry it is usual to give con- 

 siderable prominence to the difference between a mixture and a 

 compound, and to prescribe obedience to the "law of combination 

 in definite proportions " as one of the conditions which a chemical 

 compound must satisfy. The table which is given below will show 



Analysis of Oxy haemoglobin (2) . 



that this condition is very far from being fulfilled by haemoglobin, 

 for the discrepancies which appear between the various analyses are 

 too great to be accounted for by errors in the very accurate analytical 

 method of combustion. 



On the other hand the uniformity of the results yielded by 

 accurate combustions depends upon the purity of the substance 

 which is being analysed. And the difficulty of obtaining haemo- 

 globin uncontaminated by other bodies, more especially by salts and 



