586 APPENDIX 



ical laws which give results that are predictable. When material 

 is in the colloidal state, reactions do not follow the laws of mass 

 action, etc., but are dependent largely upon the amount of surface 

 exposed to the dispersion medium per gram of dispersed material 

 (i.e., specific surface) and to kinetic activity of the dispersed par- 

 ticles. The nature of the material dispersed, as well as of the dis- 

 persion medium, is also an important factor. 



Specific Surface. — The simplest example of the meaning of this 

 term would be to take material having a density the same as water, 

 i.e., at a standard temperature one cubic centimeter weighs one 

 gram. In the form of a cube, the surface area of one cube would 

 be 6 sq. cm. If this were divided into 1,000 cubes, each 1 mm. on 

 the side, the total surface would be 60 sq. cm. If the original cube 

 were divided into 1,000,000,000,000,000 cubes, eacli being 0.0001 

 mm. (0.1 micron) on the side, the total surface would be increased 

 to 6,000 sq. cm. This represents what is commonly regarded as the 

 upper limit of size of particles forming colloids. If the original 

 cube were divided into cubes each having a length of side corre- 

 sponding to the minimum of those in colloids, i.e., 1.0 millimicron or 

 0.001 micron, the total surface would be increased to 14.83 acres. 

 In this case the specific surface woukl be per gram of dispersed 

 material. 



Surface Tension.— Wells has defined surface tension as ''the 

 force with which a fluid is striving to reduce its free surface to a 

 minimum." At the surface of contact of a liquid with air there 

 is an interfacial tension and Avitliin a colloid there exist interfacial 

 tensions where the dispersion medium is in contact with the surface 

 of the dispersed material. This can, perhaps, be understood by 

 visualizing not a colloid but a drop of fat added to hot water. It 

 appears as a flattened sphere floating on the surface because the 

 attraction of the fat molecules for each other is greater than the 

 attraction of the surface water molecules for the fat. In this case, 

 part of the fat is exposed to air and part to Avater. The force or 

 tangential pull that maintains the surface of the oil droplet ex- 

 posed to air is its surface tension in respect to air, while the force 

 that maintains the integrity of the surface in contact with water 

 is its surface tension with respect to water. Thus the student 

 should be able to appreciate that at the surfaces of contact between 

 two substances there exist interfacial tensions. Ordinarilv the sur- 



