90 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



gentle heat. When heated much above this point albuminous bodies 

 then pass into the insoluble form (coagulated proteids). 



Chemical Properties. — Proteids are precipitated out of their solu- 

 tions by the following reagents : the stronger mineral acids, acetic acid 

 and potassium ferro-cyanide ; acetic acid and sodium sulphate, lead 

 acetate, mercuric chloride, tannic acid, powdered potassium carbonate 

 added in bulk to saturation, alcohol, ether, and several other substances. 



Iodine stains most proteids yellow, — a point which may aid in their 

 recognition under the microscope. 



Their presence in solution may be recognized by the following 

 processes : — 



First, by coagulation. When solutions of albuminoids are gently 

 heated, provided the amount of albumen contained is at all appreciable, 

 a firm coagulum results when the solution has been warmed up to 60° 

 or 10° C. The temperature at which coagulation occurs will vary in 

 different forms of albuminous bodies, and according to the reaction and 

 chemical characteristics of the solvent. If a small amount of a dilute 

 acid is added to a solution of an albuminous body coagulation will be 

 found to occur at a lower temperature than if the solution be neutral ; 

 while, on the other hand, the presence of a small amount of alkali will 

 prevent coagulation occurring until the temperature has been raised 

 above the point at which it occurs when the solution is neutral. If a 

 large amount of alkali be present coagulation by heat will be rendered 

 impossible. Neutral salts in small amount in albuminous solution will 

 also lower the temperature of coagulation, whether the solution be 

 faintly acid, faintly alkaline, or neutral. The coagulation of albuminous 

 bodies by heat is only possible when they are in solution, and therefore 

 seems to show that the change from the soluble to the insoluble form 

 produced by heat is not so much dependent upon the heat as upon the 

 heat combined with moisture ; for if the albumen be separated from 

 solution by evaporation below the point of coagulation, the dried albumen 

 so obtained will still possess the power of solubility in water : and yet, 

 if placed in a perfectly dry tube the temperature of the albumen may be 

 raised far above the point of coagulation without any change occurring 

 in the albumen, i.e., without its losing its power of subsequent solubility 

 in water, and of being coagulated when that solution is raised to the 

 coagulating point. 



Second: If a solution which is supposed to contain albumen is 

 acidulated with acetic acid, a few drops of potassium ferrocyanide then 

 added, and the fluid boiled, albuminous bodies will be precipitated. 



Third : If the fluid is acidulated with acetic acid and a small 

 quantity of a strong solution of sodium sulphate then added, and the 

 fluid then boiled, a firm, white coagulum will result. 



