234 Biological Chemistry. 



the dextrins, and starch. In the former case the amino- 

 acids represent the final stages of hydrolysis as in the 

 latter dextrose does ; proteoses, peptones, and the dextrins 

 are intermediary degradation products. (Compare p. 215.) 



PREPARATION OF THE SIMPLE PROTEINS. 



As the proteins possess such varied physical pro- 

 perties, the methods of isolating them from their natural 

 sources are very diverse. 



Some, such as the albumins, are soluble in water, and 

 exist in aqueous solution in various body tissues and 

 fluids. They are substances of large molecular weight, 

 and belong to the class called by Graham colloids, which 

 are incapable of passing the pores of vegetable parchment 

 owing to the large size of their molecules. By submitting 

 the solutions to dialysis that is, putting them in a mem- 

 brane of parchment or collodion or other suitable mem- 

 brane, and surrounding the membrane with pure water 

 they can be freed from the presence of " crystalloids," or 

 simpler substances, which will pass the membrane. The 

 simpler substances pass the membrane into the distilled 

 water, and distribute themselves nearly equally between 

 the outside liquid and the liquid contained in the dialysor. 

 The water outside the latter is repeatedly renewed until 

 practically all the crystalloids have been dialyzed away. 

 Albumins and globulins can be precipitated from aqueous 

 solutions by the addition of various inorganic salts ; those 

 principally employed for this purpose are ammonium sul- 

 phate, magnesium sulphate, and zinc sulphate. The exact 

 mechanism of the precipitation of complex colloidal sub- 

 stances from aqueous solutions by salts has not yet been 

 satisfactorily explained. It is probably due to an altera- 

 tion in the distribution of the water between the colloidal 

 particles and the salts. When the proteins have been 



