70 PHENOMENA, ATOMS, AND MOLECULES 



than their affinity for each other. When oleic acid is placed on water the 

 acid spreads upon the water because by so doing the CO OH can dissolve 

 in the water without separating the hydrocarbon chains from each other. 



When the surface on which the acid spreads is sufficiently large, the 

 double bond in the hydrocarbon chain is also drawn down on to the water 

 surface, so that the area occupied is much greater than in the case of the 

 saturated fatty acids. 



Qils which do not contain active groups, as for example pure paraffin 

 oil, do not spread upon the surface of water. 



The measurement of the area of water or mercury which cati be com- 

 pletely covered by a given amount of a substance affords an accurate 

 method of determining the shapes of group molecules. Thus it is found 

 that the molecules of stearic acid on a surface of water have a length of 

 about 23 X io~^ cm. and cover an area of 24 X io~^^ sq. cm. These meas- 

 urements prove that the molecules are not spherical but are much elon- 

 gated. 



This theory also affords an explanation of the mechanism by which 

 colloids are formed. If a film of closely packed oleic acid molecules covers 

 the surface of water to which sodium hydroxide has been added, OH 

 groups are adsorbed by the COOH radicals, causing an expansion of the 

 lower side of the film without a corresponding expansion of the upper 

 side. This results in the bulging of the film downwards in spots so that it 

 finally detaches itself in the form of particles, the outer surfaces of which 

 consist of COOH groups together with the adsorbed OH, while the in- 

 terior consists of the long hydrocarbon chains. 



The size of the colloidal particles is determined by the difference in 

 size between the two ends of the molecules just as the size of an arch is 

 dependent upon the relative sizes of the two ends of the stones of which 

 the arch is constructed. 



In the final publication of this paper the bearing of the shapes of mole- 

 cules on the formation of colloids will be discussed in detail. 



