56 CHEMISTRY OF THE PROTEIDS CHAP. 



brought about with alkalies instead of with acids, and there cannot 

 be any doubt that the usual method of distilling with magnesia 

 does not only yield preformed ammonia. For this reason Hart l 

 distils the mixture of dissociation-products resulting from the action 

 of sulphuric acid, with barium carbonate, and obtains ammonia in 

 smaller quantities. Dzierzgowski and Salaskin 2 have studied the 

 formation of ammonia according to the method of Nencki and Zaleski, 3 

 after peptic and tryptic digestion, and Cohnheim 4 has done the same 

 after digestion with erepsin, and the results seem to show that ammonia 

 is a primary dissociation-product as much as are the amino-acids. It 

 must be admitted that the ammonia values of dissociation-products 

 obtained by the action of acids vary according to the concentration of 

 the acid and the duration of the boiling, 5 while the definitely known 

 dissociation-products of albumins do not give off ammonia on being 

 boiled with acids. The numbers of Dzierzgowski and Salaskin further 

 agree but little with one another. We have not come as yet to the 

 end of this question. 



The amounts of ammonia which are obtained from different 

 proteids by dissociation with acids and subsequent distillation with 

 magnesia, have been determined quantitatively by Hausmann, 6 Kossel 

 and Kutscher, 7 Friedmann, 8 Henderson, 5 Osborne and Harris, 9 

 Schulze, 10 Pick, 11 and others. Most of these numbers will be found 

 in the tables on pp. 70-75. (See also p. 77.) The amount of 

 ammonia varies from 4 per cent in some vegetable' proteids to 0'4 

 per cent in gelatine. Ammonia seems to be absent only in some of 

 the protamins. 



28. Cystin, C fi H 12 O,N 2 S 2 . 



Cystin occurs in nature in two distinct forms, in the one the 

 amino-group is in the a-position = A-cystin or Protein-cystin, while 

 in the other the amino group is in the /^-position = B-cystin or 

 Stone-cystin. 12 



1 E. Hart, Zeitschr. f. physiol Chem. 33. 347 (1901). 



2 S. Dzierzgowski and S. Salaskin, ZentralbLf. Phys. 15. 249 (1901). 



3 M. Nencki and J. Zaleski, ibid. 33. 193 (1901). 



4 0. Cohnheim, Zeitschr. f. physiol. Chem, 35. 134 (1902). 



5 Y. Henderson, ibid. 29. 47 (1899). 



6 W. Hausmann, ibid. 27. 75 (1899), 29. 136 (1900). 



7 A. Kossel and F. Kutscher, ibid. 31. 165 (1900). 



8 E. Friedmann, ibid. 29. 50 (1899). 



9 T. B. Osborne and J. F. Harris, Journ. Amer. Chem. Soc. 25. 323 (1903). 



10 E. Schulze, Zeitschr. f. physiol. Chem. 25. 360 (1898). 



11 E. P. Pick, ibid. 28. 219 (1899). 



12 The author proposes to distinguish the two cystins as A and B, for both are met 

 with in proteids and in urinary calculi. 



