PREFACE 



Color in nature may properly be divided into two group?, namely, 

 color due to structure, which is caused by light reflections from col- 

 loidal particles of air or water, and color due to pigments, which is 

 caused by substances having remarkable powers of absorption of light 

 rays of certain wave length and reflection of others. The reflected 

 rays, of course, give the pigment its color. 



The present monograph treats of pigmented substances having a 

 yellow, yellow-orange, orange, red-orange and red color. So far as the 

 author is aware no authentic instances of structural colors of these 

 hues have been reported. In fact, the wave length of light in these 

 regions of the spectrum is probably too great for such a phenomenon 

 to occur for colloidal emulsions having the refractive index of air or 

 water. The particular pigments to be considered are widely dis- 

 tributed in every stage of living matter, and are perhaps more fre- 

 quently encountered than any other class of natural pigments. They 

 have attracted the attention of the biologists for at least 100 years. 

 Among the earliest inquiries were those of Caventau (1817) and 

 Goebel (1823). The former was interested in the yellow pigment of 

 the daffodil, the latter in the pigments of the crab and in the feet of 

 doves and geese. 



Active investigation of these pigments in plants and animals has 

 been confined to the past fifty years. It has only been within the past 

 fifteen years, however, that the chemical composition of any of these 

 pigments has been definitely established. Their constitution still offers 

 a fascinating problem for the organic chemist. 



The writer favors Tswett's terminology of carotinoids for these pig- 

 ments. From the standpoint of phytochemistry there is definite 

 evidence for the existence of five carotinoids, with indications that 

 several others also occur. When it was discovered that certain of the 

 carotinoids occur in animals it was believed that both plants and ani- 

 mals synthesize these pigments. It soon became apparent, however, 

 that the chromolipoids found in the higher animals, at least those 



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