INTRODUCTION 3 



indol derivatives (e.g. tryptophane and skatol acetic acid), 

 cystine (a thioserine anhydride), glucosamine, and urea. 



There are a few general reactions which are typical of all 

 proteins, and which can usually be traced to definite groupings 

 in the molecule. Amongst these is the biuret reaction : a 

 pink colour obtained by adding a trace of copper sulphate 

 and an excess of caustic soda. This is caused by the biuret, 

 NH(CO.NH 2 ) 2 radical or by similar diacidamide groups, 

 e.g. malonamide, oxamide, glycine amide. Another general 

 reaction is with " Millon's reagent/' a solution of mercuric 

 nitrate containing nitrous fumes. On warming the proteid 

 with this reagent, a curdy pink precipitate or a red colour 

 is obtained. This reaction is caused by the tyrosine group 

 (p. oxy a amido phenyl-propionic acid). Another general 

 reaction is to boil the protein with i : 2 nitric acid for some 

 days. A yellow flocculent precipitate of " xanthoproteic 

 acid " is obtained, and this dissolves in ammonia and caustic 

 alkalies with a brown or orange-red colour. Another 

 characteristic of proteins is that on dry distillation they yield 

 mixtures of pyridine C 5 H 5 N, pyrrol C 4 H 5 N, and their 

 derivatives. 



On the subdivision, classification and nomenclature of 

 the proteins much ink has been spilled, and it is impossible 

 in this volume to go into the various systems which have 

 been suggested. It should be noted, however, that some 

 writers habitually use the terms " proteid " or " albuminoid " 

 as synonyms for protein. The classification of proteins 

 adopted in this work is used because it is the most suitable 

 for a volume on industrial chemistry and has the additional 

 merits that it is simple and is already used in several 

 standard works on industrial chemistry. It is based upon 

 the behaviour of the proteins towards water, a matter of 

 obvious moment in manufacturing processes. On this basis 

 proteins may be divided into albumins, keratins and gelatins. 

 Cold water dissolves the albumins, does not affect the 

 keratins, and only swells the gelatins. The behaviour in 

 hot water confirms and elaborates the classification. When 

 heated in water, the albumins coagulate at temperatures of 



