64 THE CHEMISTRY OF CATTLE FEEDING 



low osmotic pressure, they do not crystallize readily, and, 

 under certain conditions, they are easily coagulated. In 

 short, they exhibit the characteristic properties of colloids. 

 Even colloids, however, though they offer great resistance, are 

 nevertheless regarded as electrolytes. The solutions of pro- 

 teins are not absolute non-conductors, and the compounds may 

 furnish either kations or anions. 



Coagulation. The kind of change which white of egg 

 suffers on boiling is called coagulation. Ability to undergo 

 such change is a characteristic property of the class. Each 

 individual compound coagulates at a particular temperature*. 

 For the majority of substances this temperature is considerably 

 below the boiling point of water. Compounds which are very 

 similar in other respects can often be separated in this way, 

 i.e. by fractional coagulation. 



The nature of the change is not well understood, but it has 

 been found : (i) that complete precipitation occurs only when 

 the solutions are just faintly acid. Under any other condi- 

 tions, some of the substance remains in solution. The reaction 

 of the solution changes when precipitation takes place ; neutral 

 solutions turn alkaline, and those which are acid to begin with 

 become less acid. (2) If all inorganic salts be removed from 

 the solution by prolonged dialysis, no precipitation takes place 

 on warming the solutions ; but if soluble salts be added to the 

 hot solution, the coagulated protein is immediately precipi- 

 tated. It is considered probable that the change actually 

 occurs on warming the solution, and that the products are 

 soluble in water, but are precipitated by the salt. 



The changes may be due to some slight partial decomposi- 

 tion or dehydration, or they may indicate merely some intra- 

 molecular rearrangement. 



Formic aldehyde prevents coagulation by heat. It com- 

 bines with the protein. The compound has been called 

 " methylene-albumin." Acetaldehyde acts in a similar manner, 

 but other aldehydes cause precipitation. 



Finely divided metallic silver and freshly precipitated 

 silver oxide both prevent coagulation. This phenomenon also 

 may be due to combination. The silver may displace an 



