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SCIENCE PROGRESS 



without causing the hydrocarbon chains to separate from one 

 another. 



Since the spreading of an oil is thus due to the existence 

 of an active group, it follows that oils without active groups 

 will not spread on water. This has been found to be the case 

 with hydrocarbon oils, aliphatic and aromatic. 



On the above view we conclude that in the case of oleic 

 acid films on water the carboxyl groups are immersed and the 

 hydrocarbon chains stand vertically upward. Acetic acid is 

 readily soluble in water because of the great attraction of the 

 carboxyl group for water which overcomes the attraction of the 

 alkyl groups for one another, these being fewer in number than 

 in the case of oleic acid. 



The available data indicate also that a double bond increases 

 the solubility of a compound in water. The double bond is 

 therefore attracted by the water molecules. The carboxyl 

 groups exert, however, a greater attraction upon water, and 

 when the water surface is limited the double bonds are dis- 

 placed by the carboxyl groups. Following out this line of 

 reasoning, Langmuir has made a number of determinations of 

 the spreading of various substances upon water, from which 

 he has calculated the average cross section of the molecules in 

 a plane parallel to the water surface, and likewise the length 

 of the molecule perpendicular to the surface. A few of the 

 results are as follows : 



An examination of the results shows that the cross sections 

 of the molecules vary over quite a wide range, from 21 to 

 126 x io" 16 cm 2 . The three saturated acids, palmitic, stearic, 

 and cerotic, all occupy nearly the same areas, notwithstanding 

 the fact that the number of the carbon atoms increases from 

 16 to 26. It is also evident that the molecules are much elon- 

 gated, the length of the palmitic molecule being about 5 '2 

 times its average diameter. The results indicate that the 

 molecules arrange themselves on the surface with their long 



