ELEMENTARY PHYSIOLOGICAL CHEMISTRY 173 



proteid molecule. 1 Consequently protamin, which does not contain an 

 aromatic radicle, gives neither test, and gelatine, in which there is only 

 a trace of an aromatic body gives only a very feeble colouration. 



II. Precipitants of Proteids. Certain reagents have the power of 

 throwing proteids out of solution without in any way changing their 

 chemical nature, i.e. the precipitated proteid is still soluble in its 

 original solvents. These reagents are : 



(1) Neutral Salts (i.e. neutral salts of the alkalies and of certain of 

 the alkaline earths). 



(a) Ammonium Sulphate. 



EXPERIMENT VIII. Half fill a test tube with diluted egg-albumin, 

 and add to this crystals of ammonium sulphate till no more will dis- 

 solve (i.e. until the fluid is completely saturated). A precipitate of 

 proteid is produced. Filter. Test the filtrate by any of the colour 

 reactions described above, when it will be found that no proteid is 

 to be detected. 



Add water to the precipitate on the filter paper ; by so doing a weak 

 saline solution is produced (the ammonium sulphate adherent to the 

 filter being dissolved), and in this the precipitate dissolves, the resulting 

 solution giving the colour reactions described above. 



EXPERIMENT IX. Repeat Experiment VIII. with a solution of 

 Witte's peptone (a mixture of albumoses and peptone). In this case the 

 filtrate gives the colour reactions. To obtain the biuret reaction a 

 large excess of KOH is, however, necessary, as the Am 2 SO 4 present 

 in the filtrate at first reacts with it forming K 2 S0 4 . The colour pro- 

 duced by this latter test is rose-pink, showing that the filtrate contains 

 peptone. Saturation with ammonium sulphate precipitates all proteids except 

 peptone. Anhydrous sodium sulphate at 30 C. possesses the same 

 precipitating properties as ammonium sulphate (see Advanced Course) 



(b) Magnesium Sulphate. 



EXPERIMENT X. Saturate some egg-white solution with crystals 

 of MgS0 4 a precipitate (of globulin) falls down filter the filtrate 

 gives the colour reactions. The precipitate, if dissolved by adding 



1 The formula for benzene is 



CH 



CH CH 



II I 



CH CH 



X ^> 



CH 



and if one of the *H* atoms be replaced by 'OH,' Phenol, a hydroxy-benzene, 

 results. The radicle C 6 H 5 is often called Phenyl. 



