428 PRACTICAL PHYSIOLOGY 



quantities at a time, the mixture being briskly stirred between each 

 addition. The precipitated globulin is filtered off' and the filtrate, 

 which reacts alkaline to litmus, is treated with the ammonium sulphate 

 solution drop by drop till a faint haze of precipitated albumin is 

 obtained. A drop of water is added so that the haze just dis- 

 appears. The solution is now treated with 10 per cent, acetic 

 acid, cautiously added drop by drop, until a precipitate of albumin 

 just forms. The flash is laid aside, and in twenty hours it will 

 be found that the originally amorphous precipitate has developed a 

 large number of needle-shaped crystals (see Fig. 143). 



The Colour Reactions of Proteids. Besides the three colour reactions 

 already described (viz., Millon, xanthoproteic, and the biuret) there 

 yet remains to be described ddamkiewicz's reaction. The reaction is 

 due to the presence in the proteid molecule of tryptophane (see p. 452 ). 



Mix in a test-tube two parts of glacial acetic acid, and one part of 

 concentrated sulphuric acid ; now add a few drops of a solution of egg 

 albumin, when a violet red colour will be produced. Hopkins and 

 Cole have shown that the reaction is due to. the presence of glyoxylic 

 acid, which is a common impurity of glacial acetic acid. A pure 

 solution of glyoxylic acid gives a very distinct reaction. The purest 

 forms of acetic acid do not give the reaction since they do not contain 

 glyoxylic acid. 



The Precipitants of Proteids. Sodium sulphate possesses at 30 C. 

 the same proteid precipitating powers as ammonium sulphate. It is of 

 great advantage when it is desired to estimate the amount of proteid 

 contained in any fluid. By precipitating with sodium sulphate, the 

 amount of proteid can be determined by estimating the amount of 

 nitrogen contained in the precipitate (determined by KjeldahFs method) 

 and multiplying by 6*25. When ammonium sulphate is employed, it is 

 obvious that this method cannot be used. 



Coagulation of Proteids by Heat. Native proteids are coagulated 

 by heat in the presence of neutral salts. A solution of albumin in 

 distilled water, however, does not coagulate even on boiling. If 

 blood serum be weakly acidified with acetic acid, and very gradually 

 heated in a warm bath, it will be found that a dense coagulum is 

 obtained at 75-77 C. and that, if this be filtered off, the filtrate will 

 give another, but fainter coagulum, at about 84 C. By this method 

 called fractional heat coagulation it has been attempted to identify 

 several varieties of prpteid in blood serum, but, since it is only in their 

 heat coagulation points that the fractions differ from one another, it is 

 highly improbable that they are really different varieties of proteid. 



Classification of Proteids. I. Protamines. These differ from all 



