THE BLOOD. 



221 



The most certain means of identifying blood, however, de- 

 pends on the peculiar behavior of hemoglobin toward light, 

 which will be shortly explained. 



HEMOGLOBIN. 



Composition. In the systematic classification of the pro- 

 tein bodies hemoglobin is grouped among the proteids or com- 

 pound substances, inasmuch as it may readily be broken up 

 into a fraction containing iron called hematin, and a histone 

 substance called globin. This cleavage is very easily effected 

 by the action of weak acids and in the mean the hematin frac- 

 tion is found to amount to about 4.3 per cent. In some ex- 

 periments as much as 94 per cent of globin has been recovered. 

 It is therefore likely that only the two substances are present. 

 The properties of hemoglobin are not quite constant, inasmuch 

 as from different bloods products of slightly different compo- 

 sition have been obtained. It is possible to secure the hemo- 

 globin in crystalline condition suitable for analysis. A num- 

 ber of such determinations have been made and from them 

 formulas have been calculated. These formulas can be at 

 best only more or less close approximations, but they are inter- 

 esting as illustrating the great molecular weights here con- 

 cerned. 



Analyses of Hemoglobin. Several results obtained by 

 different observers are here given. The variations must be 

 partly due to differences in methods of preparation and 

 analysis. 



In the first analysis the ratio of the sulphur atoms to the 

 iron atoms is 2 : i ; in the third analysis it is 3:1. On the 

 assumption that the molecule contains but one atom of iron 



