PHYSICAL FORCES INVOLVED IN BIOLOGICAL ACTIVITIES 173 



particle subject to these molecular movements and forces responds on 

 the whole as a football might be knocked indiscriminately about a field 

 by a group of unorganized school-boys. 



Such movements of colloidal particles are supposed to render 

 colloidal solutions more stable. This taken together with the density 

 of the dispersion means, its viscosity, the size of the particles in which 

 surface action becomes more evident, and the electric charge probably 

 accounts in large part for the permanency of the dispersoid state. The 

 velocity of the movement of particles depends upon many of the factors 

 associated with colloidal permanency. An increase of temperature 

 quickens the movement not through convection currents but by the 

 molecular activity; viscosity acts in a seeming frictional capacity; 

 the density acts as if there was a tendency to close in on the particles 

 with forces which are made effective through the multiplicity of mole- 

 cules; size apparently is much like keeping a small ball in the air as 

 compared with a large ball. 



Again, the size of particles which are subject to exact measurement 



^ 



is related to^the rapidity of their movement. Exner has made this 

 comparison :- 



Diameter of par- Velocity of particle 



ticle in y. in n per second 



i-3 2 -7 



o.Q 3-3 



0.4 3-8 



It will be seen that the smaller they are the more rapidly they move. 



Brownian motion, because of the forceful drive furnished by the 

 molecules, appears to be an important factor in diffusion and osmotic 

 bearings. 



DIFFUSION, OSMOSIS, DIALYSIS, PERMEABILITY 



If a twelve per cent, warm gelatin solution is brought in contact 

 with water of the same temperature, currents, not convection currents, 

 are seen radiating, spreading and extending from the gelatin solution 

 into the water until finally they merge with the water and are lost to 

 sight, when the entire mass becomes uniform and homogeneous. A 

 strong salt solution, when placed in the bottom of a cylinder and water 

 carefully poured above it, will little by little work up into the water 

 until the whole is one homogeneous concentration. This would also 



