52 



COLLOIDS IN BIOLOGY AND MEDICINE 



miniature (Fig. 6). By means of the ultramicroscope he counted particles in 

 each layer of 0. 12 mm. The (osmotic) pressure of a particle may be calculated 

 by the following formula applicable to the kinetics of gases: 







and n are the number of particles counted in the unit volume at the levels 

 o and h, m = mass, g = acceleration due to 

 gravity, s = density of particles. The value of k 

 proved to be 43 10~ 15 . 



If this pressure of a single particle expresses 

 the pressure of a single molecule (in solution or as 



a gas), the equation k = -rr applies. 



Here N is the number of the molecules present in 

 a gram molecule as determined by other methods, 

 that is, 6 10 23 , T = 295, k is 43 - 1Q- 15 . From 

 this we derive the value R = 2.1 cal. instead of 

 1.98 cal. From this the molecular weight of the 

 gamboge particles was proved to be 3 10 9 . 



Diffusion. 



If pure water is layered over a concen- 

 trated sugar or salt solution so that there 

 is no mixing, it is found that after a certain 

 time (hours or days), the sugar or salt 

 passes into the water; we say that it 

 diffuses into the water. If one chooses a 

 colored salt solution, e.g., copper sulphate, 

 the path of the . diffusion may be easily 

 observed by the coloration. It is in prin- 

 ciple the same process as when one per- 

 mits a compressed gas to stream into the 

 air the distinction lies only in the differ- 

 ence in speed. 



It has been shown that different sub- 

 stances have very different rates of diffu- 

 sion which are characteristic for these 

 substances. These characteristic con- 

 stants are called coefficients of diffusion. 



PIG. 6. Mastic suspension 

 showing the effect of grav- 

 ity. (Perrin.) 



The coefficient of diffusion expresses the amount of substance which 

 passes through an area 1 cm. 2 per second 1 from a solution contain- 

 ing 1 part per cc. 



1 Because this time is so brief it is usually necessary to multiply the coefficients 

 by a large factor or to choose the day as the unit of time. 



