96 THE PROTEIN SUBSTANCES. 



The precipitation of proteids and also other soluble proteins by salts 

 stands in close relation to their colloidal nature, and in this connec- 

 tion we refer to what has been said in Chapter I. The proteids do not 

 as a rule diffuse through animal membranes, or only to a very slight 

 extent, and hence have in most cases a pronounced colloidal nature in 

 GRAHAM'S sense. They belong to the hydrophile colloids; their solu- 

 tions show properties in common with those of typical colloids and also 

 true solutions. Certain of them, especially the peptones and a few 

 proteoses, which will be discussed later, seem to occupy an intermediate 

 position, as their solutions are characterized by a lesser viscosity and 

 greater diffusibility and nitration ability, are not readily precipitable 

 by alcohol or coagulable by heat, and are only partially precipitable 

 by salts. 



The solutions (or suspensions) of proteids in water, the proteid hydro- 

 sols, are converted by various means into proteid hydrogels. Of these 

 means we must specially mention the following: flocking out with salts, 

 precipitation with alcohol, gelatinization of a gelatin solution on cool- 

 ing, and coagulation by the action of enzymes or heat. 



Those proteids which occur, according to the common views, pre- 

 formed in the animal fluids and tissues, and which have been isolated 

 from these by indifferent chemical means without losing their original 

 properties, are called native proteids. New modifications having other 

 properties can be obtained from the native proteids by heating, by the 

 action of various chemical reagents such as acids, alkalies, alcohol, and 

 others, as well as by proteolytic enzymes. These new proteids are called 

 modified (" denaturierte ") proteids, to differentiate them from the native 

 proteids. 



The precipitation with alcohol is a reversible reaction, as the pre- 

 cipitate redissolves on subsequent dilution with water. The proteids 

 are changed by the action of alcohol, some readily and quickly, others 

 with difficulty and very slowly; the proteid then becomes insoluble in 

 water and is modified. 



On heating a solution of a native proteid it is modified at a different 

 temperature for each different proteid. With proper reaction and other 

 favorable conditions, for instance in the presence of neutral salts, most 

 proteids can in this way be precipitated in a solid form as coagulated 

 proteid. The hydrosol is converted into hydrogel, but as a modification 

 takes place, this process is irreversible. The temperature at which 

 coagulation occurs is a variable one for the same protein under different 

 conditions of the experiment. The various temperatures at which 

 coagulation of different proteids occurs in neutral solutions containing 

 salt have in many cases given us good means for detecting and separating 

 proteids. The views in regard to the use of these means are somewhat 



