174 PEACTICAL PHYSIOLOGY 



water, also gives the colour reactions. Magnesium sulphate in 

 saturated solution precipitates certain proteids (globulins). It does 

 not precipitate albumins. 



(c) Sodium Chloride, Ammonium Chloride. These salts in their 

 action on proteids resemble magnesium sulphate. 



III. Coagulants of Proteids. A coagulum differs from a precipitate 

 in that it is no longer soluble in its original solvents. In other words, 

 its physical or chemical nature has undergone some change. 



Coagula may be produced by the following means : 



(1) Heat. 



EXPERIMENT XL Add to about 5 c.c. of diluted egg-white a drop 

 of dilute acetic acid and boil a white coagulum is formed. 



The acetic acid is added to prevent the formation of alkali albumin, 

 which is much more easily formed than is acid albumin, and which 

 is not coagulated by heat. 



Different proteids coagulate at different temperatures globulin and 

 albumin at 75 C., fibrinogen at 56 C. and certain proteids, viz. 

 caseinogen, do not coagulate at all when heated. 



(2) Mechanical Agitation. By shaking a solution of egg albumin 

 with sand, strings of coagulated proteid like strings of fibrin are 

 deposited. 



(3) Mineral Acids and Salts. Any strong mineral acid, (HC1, HNO 3 , 

 etc.), or mineral salt, (HgCl 2 , CuS0 4 , etc.), if added to a solution of 

 proteid, will cause a coagulum to form. Certain other bodies, such as 

 tannin and picric acid, yield a similar result. 



(4) Prolonged Action of Alcohol. The addition of alcohol to a pro- 

 teid solution at first forms a precipitate, but if this be kept standing 

 for some time under the alcohol it changes into a coagulum. Pepton 

 and fibrin ferment (see blood) take longer to undergo this change than 

 other proteids, and this property is sometimes taken advantage of to 

 separate these from other proteids. 



(5) Ferments. When blood clots, the soluble proteid fibrinogen is 

 changed by the action of a ferment (fibrin ferment) into the coagulated 

 proteid fibrin. A similar change is produced in milk, the soluble 

 proteid caseinogen being changed into an insoluble modification by the 

 action of the ferment rennin. 



