GRAVITY AND CENTRIFUGAL FORCE 



333 



centrifuge tube and photographs were made after various 

 intervals of time showing the density of the solute at various 

 distances from the axis of rotation. By analyzing these photo- 

 graphs with a photo-densitometer very accurate measurements 

 of concentration were secured. Table III shows how the 

 molecular weight was calculated from the change in concentra- 

 tion with distance for one set of experiments. 



During the course of the investigation the initial concentra- 

 tion was varied from 0.5 to 3.0 gm. of hemoglobin per 100 cc. of 

 solution, the length of the column from 0.25 cm. to 0.8 cm. 

 and the speed of revolution from 7200 to 10,000 r.p.m. without 



0/ OZ 03 O.* OS 06 01 O.B 0? J.O CfTt 



r= radius of particles in millimicrons (10-' cm). 

 Fig. 1 



producing any marked change in the calculated molecular 

 weight.* 



* An important contribution to this subject has recently been made 

 by Kai O. Pedersen, Z. physik. Chem. 170A,41 (1934). It consists of a 

 study of the radial variation of the concentration of salts in aqueous 

 solution at equilibrium in a centrifugal field of force of the order of 

 2 X 10^ times the earth's gravitational field. The change in concentra- 

 tion is measured by photographing the distortion of the image of a scale 

 observed through the column of liquid rotated at a speed of 55000 r.p.m. 

 in the usual manner. From the displacement of the scale lines due to 

 the change in the index of refraction, one can calculate the radial varia- 

 tion in concentration due to the force field. A thorough discussion is 

 given of the thermodynamic relations involved, and an equation is 

 derived relating the molecular weight to the concentration changes 

 observed and the activity coefficients. The average error of the molec- 

 ular weights so determined is about ten per cent. If it is possible to 

 obtain accurate values of the absolute concentration changes this may 

 be a valuable means of calculating activity coefficients. 



