52 PHYSICAL CHARACTERS OF ORGANIC SUBSTANCES 



a colloid soil present collectively an enormous surface. 

 If we take a solid spherical object, such as an orange, 

 and cut it into halves, it is clear that we have greatly 

 increased the exposed surface, for to the original external 

 surface we have added the cut surfaces of the two 

 halves. If we now cut the halves into quarters the 

 total surface is still further increased, and the more 

 we divide the orange the greater the surface exposed. 

 Thus the collective surfaces of all the disperse particles 

 in a drop of a colloid sol are immensely greater than 

 the surface of the sphere which would be formed if 

 all the particles in the drop were aggregated in a 

 single mass. 



Surface Energy, Surface Tension and Adsorption. — 

 Now, where a surface exists, i.e. where solid or liquid 

 matter is in contact with other matter, that surface 

 is the seat of free energy, because of the break in 

 action of molecular forces at the surface, and this 

 is expressed in what is called surface tension. The 

 surface of a drop of liquid in air, for instance, tends 

 to contract, and that is why a free drop of liquid assumes 

 the spherical shape, in which the surface has contracted 

 as much as possible, and has the smallest possible 

 area in relation to the mass of the liquid. Liquids 

 with a high surface tension form drops which do not 

 easily spread upon a solid surface, because their high 

 surface tension tends to keep the drop spherical and 

 prevent it spreading. Mercury, water and alcohol 

 form a series of liquids with decreasing surface tension, 

 and a corresponding increasing readiness to " wet," 

 i.e. spread upon, a clean sheet of glass. 



The enormous collective surface, i.e. the sum of all 

 the surfaces, of the disperse particles or droplets of 

 a sol involves a corresponding amount of surface energy 



