CRYSTALLIZATION 311 



thirty minutes, produces an appearance of the white resembling 

 that of curdled milk, while seven thousand atmospheres pressure 

 brings about complete gelation. 



5. The Crystallization of Proteins. — A number of proteins, 

 particularly the vegetable proteins and hsemoglobin, are readily 

 obtainable in the form of crystals. Other proteins, such as 

 egg- and serum-albumin only yield crystals with considerable 

 difficulty. For the crystallization of these latter proteins mere 

 concentration of their solutions is insufficient; inorganic salts 

 in very high concentration must also be present (39) (41) (42) 

 (48) (94) (73). Thus egg-albumin may be prepared in a crys- 

 talline condition by adding to its solution an equal volume of 

 saturated ammonium sulphate solution and acidifying with 

 acetic acid. The coagulum which forms becomes crystalline on 

 standing. It would appear that the crystalline product is not 

 the uncombined protein but a salt of the protein, formed with 

 an acid and also, probably, with the ammonium sulphate (Cf. 

 Chap. VI) (59). 



The protein crystals are optically true crystals. According to 

 Wichmann all albumin-crystals are either crystallographically 

 identical or else isomorphic. They very readily absorb impuri- 

 ties (106) (45) (95), and the circumstances of their preparation 

 involve the occlusion of a considerable quantity of the mother 

 liquor. Hence repeated recrystallization is required to remove 

 from them even colloidal impurities (95). Contamination by 

 crystalloidal substances, combined or physically associated, 

 obviously cannot be avoided, and these must be subsequently 

 got rid of by very prolonged dialysis of the dissolved crystals. 



A monumental and most fundamentally important contribu- 

 tion to our knowledge of the crystallography of proteins has 

 been furnished by the investigations of Reichert and Brown (82) 

 on the relationship of the morphology of hsemoglobin crystals 

 to the biological classification of the species from which it is 

 derived. From an enormous number of measurements of crystal 

 angles, etc., conducted upon haemoglobins derived from a large 

 variety of species, these investigators conclude in the first place 

 that the crystals of the species of any genus belong to the same 

 crystallographic system and generally to the same crystallo- 

 graphic group, and they have approximately the same axial 

 ratios, or their ratios are in simple relation with each other. In 



