ALBUMINS OR PROTEINS. 145 



horse produces, therefore, a pigment, but it is not utilized. This pigment, 

 called hippomelanin, has been very exhaustively investigated. 1 It occurs 

 as a very finely divided, brownish-black powder. No individual constit- 

 uent of melanin has so far been definitely isolated. The constituents of 

 the other melanins, prepared from the hair, choroidea, etc., has also never 

 been cleared up. The melanins are, undoubtedly, not simple substances. 

 It is difficult to obtain any exact knowledge about these substances, 

 because they are not readily purified. They are extremely resistant to 

 acids and alkalies, and to oxidation and reduction processes. Some will 

 dissolve in alkali; others will not. Some contain iron; in others, this ele- 

 ment is lacking. It has been suggested that, because some of the melanins 

 contain iron, they are related to the coloring matter of the blood. It 

 is possible that some of these pigments may trace their origin back to 

 hematin, although no proof has been presented to substantiate this 

 hypothesis. The melanins are characterized by a high carbon and a low 

 hydrogen content. Many of them have considerable sulphur in their 

 composition. 



By the hydrolysis of almost all the albuminous bodies with acids, 

 products are obtained which very much resemble the naturally-occurring 

 melanins. These black products are called humin substances. They are 

 supposed to be related to the natural melanins, and the suggestion has 

 been made that glucosamine, tryptophane, tyrosine, and lysine participate 

 in the formation of pigment. We know nothing definite 2 about the 

 constituents of these humin substances, and are unable to decide whether 

 there is any distinct relationship between these and the melanins. We 

 will particularly emphasize the fact that the hypothesis just mentioned 

 to the effect that the humin substances are built up of the fundamental 

 constituents of the albumins, is entirely without empirical justification. 



We have now r mentioned the most important classes of the remarkably 

 diversified group of the proteins, or albuminous substances. The unsatis- 

 factoriness of the whole system of classification is evident from our pre- 

 sentation of the subject. It only serves as a temporary basis for further 

 orientation. It is highly important that we should sharply and distinctly 

 emphasize how slight the proofs are in the case of most of these substances 

 that we are dealing with simple substances rather than with mixtures, 

 and how extraordinarily cautious we must be for this reason in passing 

 judgment upon the results, which must eventually be referred to more 

 physico-chemical investigations. 



1 J. Berdez and M. Nencki: Arch. exp. Path. Pharm. 20, 346 (1885). M. Nencki 

 and N. Sieber: ibid. 24, 17 (1887). 



2 Cf. also Fr. Samuely: Hofmeister's Beit. 2, 355 (1902) 



