CHEMICAL CONSTITUTION AND PHARMACOLOGICAL ACTION. 41} 



stitution but have the same molecular weight, yet their affinities 

 are different. 1 



There is probably no doubt that those properties of organic sub- 

 stances which interest us as therapeutists are constitutive in nature. 



R. Meyer has published a most interesting article on certain re- 

 lations between fluorescence and chemical constitution. In this he 

 calls attention to the fact that the relations between the color of 

 chemical combinations and their constitution have not up to the 

 present time been studied with the exactness with which charac- 

 teristics less apparent have been examined, such as rotation and 

 the refractive index. The reason for this is that the refractive index 

 of a body is a definite number, the specific rotation an angle whose 

 size can be exactly determined, whereas color is more qualitative 

 in character, and, strictly speaking, is not a physical but a physio- 

 logical characteristic. A body which possesses strong ultraviolet 

 absorption bands is colorless to our eyes, yet it may appear colored 

 to a visual organ differently constituted than ours. We see, therefore, 

 that even in so conspicuous a property as color the physiological 

 factor interferes with our gaining a clear insight into the relations 

 existing between constitution and action. It will at once be con- 

 ceded that this is true to a still greater degree in the complex processes 

 which underlie pharmacological action. 



But it is just because of this intermediate position that the chem- 

 istry of dyestuffs affords so good a point of vantage for our con- 

 sideration. 1 may therefore perhaps be permitted to briefly outline 

 what has thus far been learned concerning the relations between 

 color and constitution, especially in view of the fact that I shall 

 frequently have to touch on the biology of dyes in the succeeding 

 chapters. 



In 1868 C. Graebe and C. Liebermann demonstrated that color 

 was in some way associated with a certain denser combination of the 

 atoms. Jf this is overcome by the addition of hydrogen the color 

 will disappear, the dye passing into the "leuco" combination (thus 

 indigo into indigo white), out of which it can again be produced by 

 oxidation. 



A great advance was then made by 0. N. Witt, who showed that 

 the color properties of a dyestuff are due to the presence of a certain 

 unsaturated group of atoms which he terms the color-producing or 



' Ostwald, Grundriss der allgemeinen Chemie. 



