44 FUNDAMENTALS OF SUBMICROSCOPIC MORPHOLOGY I 



tact with which the surface tension has been measured. The data given 

 in the literature usually refer to the surface tension against air. In 

 cytology, however, we are concerned in the first place with the surface 

 tension of the protoplasm against the nutrient solution or the cell 

 sap(TableXXI, p. i66). 



The surface tension against air has become of great importance in 

 physiology. As shown by analysis of foams, many substances are 

 accumulated at the surface, which usually lowers the surface tension 

 to a considerable extent (Table V), On the basis of thermodynamics 

 the Gibbs-Thomson theorem renders account of this phenomenon by 

 the two following rules : i . Substances which lower the surface tension 

 of water accumulate at the surface; 2. a small amount of a solute can 

 strongly reduce the surface tension but cannot appreciably increase it. 



Hydrophily and Upophily. To-day these relations can easily be under- 

 stood quahtatively with the aid of simple rules on the mutual mis- 

 cibility of different types of molecules. Water and ethyl alcohol, 

 for instance, are miscible in any proportions as are also absolute 

 alcohol and ethyl ether. Water and ethyl ether, however, are only mis- 

 cible to a very small extent. The phase theory contents itself with 

 determining the range of miscibility, without being concerned with 

 the cause of the insolublity. The theory of structure, however, tries 

 to form a notion of the limited solubility of water and ether and vice 

 versa. The reasoning is as follows. 



If alcohol and water are mixed, the water molecules will be pre- 

 ferably attached to the kindred hydroxyl groups, more or less accord- 

 ing to the scheme of Fig. 41a. In the presence of an excess of water 

 the OH-group is hydrated in much the same way as in Fig. 18 (p. 19) 

 by orienting and attracting the dipoles, be it only to a small extent. 

 Each OH-group, therefore, is surrounded by a water shell designated 

 by the dotted circle in Fig. 41a. The alkyl group, on the other hand, 

 tries to escape from the water molecules, because it is hydrophobic. 

 It therefore protrudes from the hydration layer if its size allows, as, 

 e.g., in butyl or amyl alcohol. In ethyl alcohol, however, the sphere 

 of action of the OH-group corresponds approximately to the length 

 of the alkyl group, hence water dipoles can settle all round the mol- 

 ecule. This explains the unlimited miscibility of ethyl alcohol and 

 water. In the higher members of the aliphatic alcohol series, however, 

 the lipophilic part of the molecular chain predominates, with the 



