CHAPTER 11* 



METHODS OF INVESTIGATION 



1. INTRODUCTION 



The substances with which this book is concerned are all of an 

 exceedingly complex character, so much so that their chemical con- 

 stitution has been determined only comparatively recently, or 

 remains still unknown. This fact, together with the profound physi- 

 ologic importance of these substances, has led to the use of a variety 

 of physicochemical methods for their investigation, producing results 

 unobtainable by purely chemical means. 



That such methods may be used is due to the fact that hematin 

 compounds and their derivatives possess a number of remarkable 

 properties. All absorb radiant energy, often in comparatively narrow 

 regions of the spectrum, the position and intensity of the maxima 

 of these absorption bands being characteristic constants for each 

 substance. Some re-emit the absorbed radiation in the form of 

 fluorescence in the visible spectrum. The majority contain acidic 

 and basic groups, either in the porphyrin group or in nitrogenous 

 compounds coordinated with the iron atom or in both. The iron atom 

 may assume both the ferrous and the ferric state, giving rise to oxida- 

 tion-reduction potentials; it also confers on all those compounds in 

 which it occurs magnetic properties which vary with the nature of 

 the linkage between it and the attached groups. Finally, those com- 

 pounds which contain protein lend themselves to methods specific 

 to this class of substances. 



We shall not discuss in detail the technique of the various methods 



*Coiitributed hy J. P. Callajjliaii. 



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