590 



APPENDIX 



"greasy" ends were turned outward in the surface which had 

 cooled in contact with the air. 



Molecular Orientation. Harkins' Theory. — This surface 

 orientation of molecules is part of Harkins ' orientation theory. He 

 would say that on the surface wet l)y water, the molecules were 

 oriented at right angles to the surface with a polar group attached 

 to a short carbon chain and that this polar group attacted the 

 water so strongly that it tended to drag the hydrocarbon chain out 

 into the water. This affinity for water was responsible for the 

 wetting. On the side exposed to air, the other end of the chain 

 was outside and it had no affinity for water, hence, the surface 



Air 



uOjj616 6ll6lloL(5 



Water 



Fig. 26. — Orientation of molecules of an alcohol (or an organic acid) at the 

 surface of its aqueous solution. From "Atoms, Ions, Salts and Sui-faces," by 

 William D. Harkins in Neicer Knotoledge of Bacteriology and Immunology edited 

 by E. O. Jordan and I. S. Falk. Reprinted by permission of the University of 

 Cliicagd Press. 



could not be wetted. Langmuir has studied this plienomenon of 

 molecular orientation and arrived at the same conclusions as Har- 

 kins. This surface orientation of molecules is graphically illus- 

 trated ])y Harkins. (Fig. 26.) 



Attraction of Colloids for Each Other. — Cohesion, Adhesion 

 and Precipitation. — Graham noted that colloids adhere to each 

 other with great tenacity. Within a colloid there are powerful 

 residual forces that hold adjacent molecules together. At the 

 surface there are unsatisfied fields of force that make it possible for 

 the dispersed particles to stick together if the forces liolding them 



