300 PRACTICAL PHYSIOLOGY 



not actually having the same composition as any known peptone (see 

 later), have many resemblances to peptones. 



The proteins have for the most part many physical and chemical 

 reactions in common. 



THE PHYSICAL AND CHEMICAL PROPERTIES OF PROTEINS. 



I. Solubility. All proteins 1 are insoluble in alcohol and ether. They 

 vary as to their solubility in water, the more common proteins 

 (albumins and globulins) being soluble in water (albumins) or in weak 

 saline solutions (globulins). Some, however, are not soluble, even in 

 concentrated saline solutions. 



EXPERIMENT IV. From the undiluted egg-white provided prepare 

 a solution of egg albumin by adding 10 volumes of distilled water and 

 mixing thoroughly in a flask. An opalescent solution is thus obtained, 

 the opalescence being partly due to the colloidal nature of the solution, 

 although in part to some other protein (ovo-mucin), which has not 

 gone into solution. This can be removed by filtering through fine 

 muslin. Note that this solution, like all colloidal solutions, gives a 

 persistent froth on shaking. 



The solution prepared above can be used in the subsequent experi- 

 ments, unless otherwise stated. 



II. Diffusibility. As the proteins give only colloidal solutions, these 

 solutions will not dialyse, that is, diffuse through animal membranes or 

 parchment paper. In this they are unlike crystalloids, such as inorganic 

 salts, which readily diffuse through such membranes. Of the various 

 forms of dialyser, a tube of parchment is the simplest. 



EXPERIMENT V. Place a mixture of diluted blood and of a 10 % 

 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 (albu- 

 mins and globulins) coagulate when their solutions are heated. Dif- 

 ferent proteins coagulate at different temperatures, varying usually 

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

 much neutral salt greatly favour heat coagulation. 



vegetable proteins are soluble in alcohol. 



