CH. xxv.] THE PROTEIDS. 395 



seeds, and in the somewhat similar granules occurring in the 

 egg-yolk of some fishes and amphibians. By appropriate methods 

 these have been separated and re-crystallised. Further, egg- 

 albumin itself has been crystallised. If a solution of white of 

 egg is diluted with an equal volume of saturated solution of 

 ammonium sulphate, the globulin present is precipitated and is 

 removed by filtration. The filtrate is now allowed to remain some 

 days at the temperature of the air, and as it becomes more con- 

 centrated from evaporation, minute spheroidal globules and 

 finally minute needles, either aggregated or separate, make their 

 appearance (Hofmeister). Crystallisation is more rapid if a little 

 acetic or sulphuric acid is added (Hopkins). Serum albumin 

 (from horse and rabbit) has also been similarly crystallised 

 (Giirber). 



Action on Polarised Light. All proteids are levo-rotatory, 

 the amount of rotation varying with individual proteids. 



Coloxir Reactions. The principal colour reactions by which 

 proteids are recognised are the following : 



(1) The xantho-proteic reaction ; if a few drops of nitric 

 acid are added to a solution of a proteid like white of egg, 

 the result is a white precipitate ; this and the surrounding 

 liquid become yellow on boiling and are turned orange by 

 ammonia. The preliminary white precipitate is not given by 

 some proteids like peptones ; but the colours are the same. 



(2) Milton's reaction. Millon's reagent is a mixture of mercuric 

 and mercurous nitrate with excess of nitric acid. This gives a 

 white precipitate with proteids which is turned brick-red on 

 boiling. This reaction and the preceding (xanthroproteic) depend 

 on the presence in proteids of aromatic radicles. 



(3) Copper sulphate, or Piotroivski's test. A trace of copper 

 sulphate and excess of strong caustic potash give with most 

 proteids a violet solution. Proteoses and peptones, however, give 

 a rose-red colour instead ; this same colour is given by the 

 substance called biuret ; hence the test is generally called the 

 biuret reaction. This name does not imply that biuret is present 

 in proteid ; but both proteid and biuret give the reaction because 

 they possess a common radicle, probably CONH. 



Biuret is formed by heating solid urea ; ammonia passes off and leaves 



2 CON 8 H 4 - NH 8 = C 8 O a N 8 H s 

 [Urea] [Ammonia] [Biuret] 



4. Adamkiewicz reaction (Hopkins' modification). When a solu- 

 tion of proteid is added to a dilute solution of glyoxylic acid, 

 and then excess of sulphuric acid is added, an intense violet colour 

 is obtained. 



