PROPERTIED .\\D METHODS <)! IDENTIFICATION 243 



tered in a crystalline condition in such (issues, as it has been stated 

 to occur in plants, the balance of the argument is at present against 

 tin- occurrence of the pure pigments. 



With regard to the lipochrome eolor reactions the results which 

 have been secured are not encouraging as to their applicability to the 

 carotinoids which occur in animal tissues. The chemistry of the 

 blue color reaction with con. H 2 S0 4 , which is given by a number of 

 aromatic substances besides the carotinoids, is not known. A positive 

 reaction with this reagent can not therefore be regarded as conclusive 

 proof of the presence of carotinoids, although it would indicate this 

 possibility. This reaction, however, fails completely in the presence 

 of glycerides, and may be vitiated even by the presence of other 

 lipoids which are attacked by strong sulfuric acid. It can not be used 

 at all for detecting carotinoids. dissolved in fats. A negative test 

 on even more suitable material is not necessarily conclusive; for 

 example, Sehrt (1904) discredited the corpus luteum pigment as a 

 lipochrome because it was colored only a faint blue by H 2 S0 4 , and 

 sometimes not at all. The reaction with H 2 S0 4 , therefore, has only 

 a very limited application to carotinoids which may be encountered 

 in histological sections of animal tissues, at least under the conditions 

 in which it has been employed up to the present time. 



Practically the same conclusion must be drawn for the use of the 

 iodine reaction. The cause of this reaction is the blue iodine deriva- 

 tive which is formed by all the known carotinoids. The reaction 

 was discovered by Schwalbe (1874) as an apparently typical pig- 

 ment reaction for the colored oil drops in the retina of certain ani- 

 mals. Since that time it has been generally held that animal pig- 

 ments which fail to give the iodine reaction are not typical lipo- 

 chromes. However, even this reaction, which possesses a firm chemi- 

 cal basis, has frequently failed for pigments which we now know 

 to be true carotinoids. For example, Kiilme (1878) failed to secure 

 the reaction with egg yolk pigment, even after isolation, and Sehrt 

 (1904) found that the corpus luteum pigment only occasionally re- 

 acted. 



These results seem very discouraging. We have already seen, how- 

 ever, that no great difficulty attends the use of color reactions in the 

 microchemical identification of carotinoids in plant tissues. The 

 author is not aware of any attempt to apply the Molisch microchemi- 

 cal method of crystallization of carotinoids to animal tissues. Un- 

 fortunately the high concentration of alkali in the Molisch reagent 



