572 CHEMISTRY OF THE PROTEIDS CHAP. 



alkalies, and therefore a Papin's pot has always been used for separat- 

 ing the two constituents of silk from one another. E. Fischer and 

 Skita have shown that fibroin may be heated for hours to 117-120, 

 provided care is taken to keep the reaction exactly neutral ; in the 

 presence of acids or alkalies it is, however, considerably changed under 

 these conditions. 



The analysis of fibroin by Cramer, Weyl, and Vignon show a low 

 carbon percentage and a high nitrogen percentage, namely, 19 per 

 cent ; the sulphur has not been determined. The dissociation-products 

 obtained by E. Fischer and Skita have been given on p. 73, No. 29. 

 Fibroin differs in its constitution considerably from other albumins, as 

 it contains more than 50 per cent of glycocoll and tyrosin, and 10 per 

 cent of tyrosin, while leucin is only present in small amounts ; glut- 

 aminic and aspartic acids are absent. The basic radicals are slight in 

 amount. Fibroin gives the biuret and Millon's reactions, 1 and, accord- 

 ing to Krukenberg, 2 also that of Adamkiewicz. It is not attacked by 

 either pepsin or trypsin, according to Weyl, while by strong alkalies 

 and acids it is converted into albumoses or albuminates. 



The lids covering the compartments in which wasp-embryos are 

 hatched, Engel 1 has found to give the reactions and to show the 

 solubilities of fibroin, and Schlossberger 3 states that cobwebs are also 

 composed of fibroin. 



Silk glue, according to the authors mentioned above, and according 

 to Bondi, 4 resembles ordinary gelatine in its solubility, but it does 

 not gelatinise so readily, and is further precipitated by acids. Its 

 dissociation-products show, however, that it is entirely different from 

 ordinary gelatine (see p. 73, No. 30). Glycocoll, if present at all, 

 only occurs in traces, while tyrosin is very abundant, and serin is also 

 met with in large amounts. Serin was first discovered by Cramer in 

 silk glue. Wetzel obtained the bases in ample amounts. Positive 

 results are obtained with the tests of Millon and Molisch. 



5. Spongin, Conchiolin, etc. 



forms the framework of the bath-sponge. It was first 

 investigated by Posselt 5 and Croockewit, 6 and later by Stadeler, 7 who 



1 W. Engel, Zeitschr.f. BioL 27. 374 (1890). 



2 F. C. W. Krukenberg, ibid. 22. 241 (1886). 



3 J. Schlossberger, Liebig's Annalen, 110. 245 (1859) 



4 S. Bondi, Zeitschr. f. physiol. Ghem. 34. 481 (1902). 



5 L. Posselt, Liebig's Annalen, 45. 192 (1843). 



6 J. H. Croockewit, ibid. 48. 43 (1843). 



7 G. Stadeler, ibid. 111. 12 (3859). 



