APPENDICES 653 



since the greater the concentration the more molecules hit the containing 

 membrane, and the higher the temperature the greater the velocity, and 

 consequently the greater the force with wliich they impinge on the 

 membrane. 



The osmotic pressure may be very higli. A 10 per cent, solution of 

 cane sugar has an osmotic pressure of over ten atmospheres. 



If a solution be separated by a semijiermeable membrane from pure 

 water or a less concentrated solution, water passes from the pure water to the 

 solution or from the dilute to the more concentrated solution. Osmosis, there- 

 fore, always occurs towards the more concentrated solution. On the other 

 hand, if the molecules of the solute are free to pass through the membrane 

 the passage is from the concentrated to the dilute solution, the tendency 

 being to establish a uniform mixture (see Diffusion). Dialysis, therefore, 

 always occurs towards the more dilute solution. 



Surface Tension. 



In the body of a fluid the molecules are equally attracted on all sides 

 by other molecules, and the resultant is zero. At the surface, ho\vever, 

 the attraction is one-sided — towards the liquid. The molecules are there- 

 fore subjected to unbalanced forces, and the surface is consequently in a state 

 of tension. It is this tension that makes a drop spherical. 



The same state of tension exists at the interface of two fluids. If the 

 fluids are miscible a solution occurs and there is no interface. If, however, 

 the fluids are immiscible an interface exists (see Colloids) and surface 

 tension is present. In a colloidal state formed by two immiscible fluids, 

 there is an enormous extent of surface (p. 13). Surface tension plays an 

 extremely important part in many physiological processes. 



The surface tension at the interface between immiscible fluids or 

 between a solid and a fluid is lowered by the presence of substances 

 in solution. 



Adsorption. 



According to the second law of thermo-dynamic, any process that 

 diminishes free energy always tends to take place. Surface tension is 

 diminished by substances in solution. These, therefore, tend to concentrate 

 at the interfaces where the tension exists. This depositing of a solute at 

 a surface of contact is termed Adsorption. In the colloidal state where 

 there is a great extent of .surface, adsorption is of great physiological 

 importance. It facilitates chemical reaction between the substance 

 composing the dispersed phase and those in solution in the continuous 

 phase. 



Thermopile. 



In a circuit composed of two difl'erent metals, if one of the junctions of 

 ! metals be at a different temperature from the other junction, a current 



