i66 



ATOMS, IONS, SALTS, AND SURFACES 



give a monomolecular film which is in equilibrium with the solution is considerable — 

 more than thirty minutes in the case of decylic acid (lo carbon atoms) (see Fig. 19). 



Fig. 18. — Areas per molecule (X-axis) for monomolecular films on water. The F-axis represents 

 the "force of compression" which is measured by the film balance, and is equal to the surface tension 

 of water minus the surface tension of a water surface covered by a film of the organic substance. The 

 curves for palmitic acid, and for stearic acid alone, are of the more usual t>'pe. These films are under 

 high pressures. Under low pressures the curves are similar to the p, v curves for gases. For com- 

 parison, two curves for polymolecular films of phenanthrene and triphenjdmethylcyanide are given. 

 The areas per molecule for these two substances are from 2 to 6 A, which is too small an area for a 

 monomolecular film. The line for palmitic acid extrapolated to zero pressure gives 20.2 A, and for 

 stearic acid on water alone the two lines give 19.1 and 19.2 A of area. Laurie acid forms a dilute 

 monomolecular film. 



