PHYSICAL FORCES INVOLVED IN BIOLOGICAL ACTIVITIES 175 



less dense. This action increases with the temperature and is pro- 

 portional to the concentration of the solution. In the illustration of 

 diffusion above by means of gelatin and salt, diffusion follows its natural 

 course; but in the case of oxygen penetrating litmus agar or any other 

 medium the action may be regarded as modified diffusion or osmosis in 

 which the medium acts as a barrier to the medium-content but allows 

 the gas (air) in its drive onward to pass and diffuse throughout. This 

 leads to the significance of permeability of membranes. 



Much attention has been given to the study of membranes as 

 they relate so closely to the membranes of cells which are concerned 

 with living processes. It is more or less simple to demonstrate 

 the passage of water and the restraining of a substance like sugar 

 by means of parchment paper. This is a common experiment. In a 

 thistle tube with its mouth covered with parchment paper place a 

 sugar solution to the neck. When plunged into water, the water will 

 pass in and appear in the rising line. At the same time no sugar 

 passes through and out into the water. The molecules appear too 

 large to pass through the pores. This membrane is semi-permeable 

 since it permits water to pass but restrains sugar. A membrane or 

 anything which does not allow anything to pass, as glass, would be 

 called impermeable. 



Whether dialysis (passage through a membrane in the separation 

 of colloids and crystalloids) or the permeability of membranes is 

 traceable to its sieve-like nature, its chemical reaction, or to its solvent 

 action or to more than one of these is a mooted problem of prime 

 interest but out of place in this consideration. Some data throwing 

 light on the action of membranes may be helpful, however. The 

 Bechhold ultra-filters made of collodion, which may be graded to vary- 

 ing porosities, have been employed in such a manner as to illustrate the 

 permeability of membranes. Some substances will pass while others 

 will not until the size of pores are adjusted. The membrane resulting 

 by the contact of potassium ferrocyanide with copper sulphate allows 

 water and potassium chloride to pass while it withholds potassium sul- 

 phate and other salts. In nature membranes may be permeable to 

 certain salts at times and impermeable at other times. Osterhout 

 has demonstrated many of the possibilities of protoplasmic permea- 

 bility. Speaking in very general terms, permeability as manifested in 

 living cells and measured by electric conductivity, as has been the case 



