560 SCIENCE PROGRESS 



being by no means amenable to experimental test. Less 

 advance has been made indeed in capillarity than in almost 

 any other branch of physics or chemistry. Considerable 

 interest attaches therefore to some quite recent attempts to 

 deal with capillary effects from a new point of view. It is 

 too soon to say whether the new mode of treatment will carry 

 us far enough. It has already accomplished enough, however, 

 to warrant a serious consideration. 



The new view, which may be called the " chemical " theory 

 of capillarity, to distinguish it from the purely physical view 

 referred to above, rests on the assumption that capillary effects 

 are essentially due to valency, residual valency. That is, the 

 forces of attraction instead of being uniform in all directions 

 are located in certain regions. These forces extend out from a 

 number of definite positions on a single molecule which is 

 thereby attached in a certain way to neighbouring molecules. 

 This mode of attachment involves a more definite conception 

 of the spatial structure of a molecule than is required by the 

 older physical view, and herein indeed lies one of its principal 

 advantages. The new concept of capillarity has been advanced 

 independently by two American investigators, Langmuir (Met. 

 Chem. Eng. 15, 468, 191 6 ; Journ. Amer. Chem. Soc. 39, 

 1848, 191 7) and Harkins (Journ. Amer. Chem. Soc. 39, 354, 

 541, 1917). We shall give a very brief summary of some of 

 the results arrived at, following in the main the work of Lang- 

 muir. 



According to the new view of capillarity the structure of 

 the surface layer of atoms is regarded as the principal factor in 

 determining the magnitude of the surface tension of liquids. 

 The molecules arrange themselves in the surface layer in such 

 a way that their active portions, i.e. the portions possessing 

 much residual affinity, are drawn inwards, leaving the less active 

 portions of the molecules to form the surface layer. The force 

 is regarded as electromagnetic in nature, and the arrangement 

 is such as to make the stray electromagnetic fields a 

 minimum. 



The surface energy of a liquid is thus not a property of 

 the molecule as a whole, but depends only upon the least active 

 portions of the molecules, and the manner in which these are 

 able to arrange themselves in the surface layer. 



In liquid hydrocarbons of the aliphatic series the molecules 





