330 



PHYSICAL PROPERTIES 



4. The Diffusion of Proteins in Solution. — It was pointed out 

 by Graham in his classic memoirs on diffusion, that the proteins, 

 like other colloids, are very sparingly diffusible. Nevertheless 

 they diffuse through water, albeit with the extreme slowness 

 indicated by their relatively enormous molecular weight. From 

 the law of Fick (10) (30) we have: 



ds = Dq^£dz, 



where ds is the quantity of diffusing substance which passes in the 

 time dz through a diffusion cylinder having the cross-section q, 

 c is the concentration of the substance in the whole cross-section 

 at the point x, c -{- dc is the same quantity at the point x -{- dx 

 and D is a constant peculiar to the substance, expressive of its 

 diffusibility, and known as the "diffusion coefficient." 



The following are the diffusion coefficients of certain proteins, 

 estimated by the observers named and cited after Wo. Ostwald 

 (81) (45) (46). For the purpose of comparison the value of D 

 for sodium chloride is placed at the head of the list. 



Substance 



Sodium chloride. 

 Clupein sulphate 

 Egg-albumin. . . . , 



Ovomucoid , 



Albumin 



Observer 



Voigtlander 



Herzog 



Herzog 



Herzog 



Graham-Stefan 



D 



1.040 

 0.074 

 0.059 

 0.044 

 0.063 



Temperature, 

 degrees 



20 

 18 

 18 

 18 

 13 



Now it has been shown by Nernst (73) and Planck (82) that the 

 driving force which produces diffusion is osmotic pressure. Hence 

 we can conclude that the proteins, in solution, exert a definite, 

 although small osmotic pressure. 



From the diffusion coefficient of egg-albumin, Herzog and 

 Kasarnovsky (46) have calculated that it exerts an osmotic 

 pressure corresponding to a molecular weight of about 20,000. 

 That proteins in solution exert a definite osmotic pressure is also 

 shown by cryoscopic, ebullition, and direct measurements, of 

 which a brief description will be found below. 



It has been found by Dabrovsky (23) that the diffusion-coeffi- 

 cient of crystallized egg-albumin is decidedly greater when the 

 diffusion takes place into a solution of ammonium sulphate than 



