CH. XXV.] 



PROPERTIES OF PROTEIDS 



397 



and globulin remain inside ; llie globulin is, however, precipitated, as 

 the salts which previously kept it in solution are removed. 



Crystallisation. Haemoglobin, the red pigment of the blood, is 

 a proteid substance and is crystallisable (for further details, see The 

 Blood, Chapter XXVI.). Like other proteids it has an enormously 

 large molecule ; though crystalline, it is not, however, crystalloid in 

 Graham's sense of that term. Blood pigment is not the only 

 crystallisable proteid. Long ago crystals of proteid (globulin or 

 vitellin) were observed in the aleurone grains of many 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 wriite of egg is diluted 

 with an equal volume of saturated solu- 

 tion 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 concen- 

 trated from evaporation, minute spheroidal 

 globules and finally minute needles, either 

 aggregated or separate, make their appear- 

 ance (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 pro- 

 teids are levo-rotatory, the amount of 

 rotation varying with individual proteids. 

 Several of the compound proteids, e.g., 

 haemoglobin, and nucleo - proteids are 

 dextro-rotatory, though their proteid components are levo-rotatory 

 (Gamgee). 



Colour 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 pre- 

 cipitate 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 



FIG. 341. Dialyser made of a tube 

 of parchment paper, suspended 

 in a vessel through which water 

 is kept flowing. 



