Outline of recent. developments

Indications of further complexities in the coherent type of surface films were found by Langmuir, and have been further investigated by Labrouste,1 Adam, and others. It is now well established that there frequently exists a coherent, expanded state in insoluble surface films of fatty substances, intermediate in area between the very closely packed condensed films, and the gaseous films in which the molecules move about independently on the surface. This most interesting type of film will be discussed in 18 and 19. 

Although the fact that surface films affect the contact potential between the liquid and the air has been known for some time, Guyot (1924)2 was the first to make systematic measurements of the effect of surface films of insoluble fatty substances on this potential. Frumkin (1925)8 continued this and Schulman and Rideal (1931)4 undertook a detailed study of several types of insoluble film, comparing the measurements of surface potential5 

Zocher and Stiebel (1930) showed how dark ground ultramicroscopic examination of a surface film yields invaluable information as to the extent to which the material is spread on the surface ( 8). 

Finally, electron diffraction is now available as a method for examining surface films of non-volatile substances on liquids which can be placed in a high vacuum, such as mercury it does not, however, appear to have been much used on liquids, though on solids it is now one of the most important experimental methods. 

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