194 



PRACTICAL PHYSIOLOGY 



cent, sodium chloride solution in the dialyser provided. Test a 

 sample of distilled water with silver nitrate, and note that no haze 

 of silver chloride occurs. Place the dialyser in a beaker of this 

 water and allow dialysis to proceed for a day. On now testing the 

 water in the beaker for chlorides with silver nitrate, it will be found 

 that a white precipitate of silver chloride occurs, showing that the 

 chlorides have diffused through the parchment. It can be shown, 

 however, that no protein has dialysed through, by the, absence of 

 pigment and by applying the tests for protein given below. 



III. Heat Coagulation. Most of the so-called native proteins 

 (albumins and globulins) coagulate when their solutions are heated. 



FIG. 172. Crystallised albumin. X 600. 



Different proteins coagulate at different temperatures, varying 

 usually from 56 C.-78 C. A faint degree of acidity and the 

 presence of a neutral salt greatly favours heat coagulation. 



EXPERIMENT. Fill a narrow glass tube with some egg-white solution, 

 faintly acidulated with acetic acid, and seal off one end. Now fix this to the 

 lower end of the thermometer by means of small elastic bands. Gradually 

 heat in a test tube placed in a water bath and observe the temperature at 

 which the albumin becomes opaque and set. 



IV. Crystallisation. Most proteins crystallise with difficulty ; 

 the blood pigment of certain animals, however, crystallises readily. 

 (See later under Blood, Chapter XI.) Egg albumin and serum 



