148 FINE-STRUCTURE OF PROTOPLASM II 



are present, the possibility exists of their being attracted electro- 

 statically by the local negative charges of the dipole groups of neigh- 

 bouring molecules. To a certain extent the hydrogen atom acts as an 

 intermediary between the two molecules and connects them by forming 

 some kind of bridge. This is represented in Fig. 98 for two poly- 

 peptide molecules running in opposite di- 



c^o H- N rections. The hydrogen atom is ifted some- 



RHC CHR what out of its position in the original 



N-H o^c molecule and it looks as if part of the hy- 



O'^c N H- drogen valency is transferred to the neigh- 



CHR RHC bouring molecule. Clearly, this schematic 



H-N c-^o representation of the "secondary valencies" 



9^'^ >^-i^ gives only a very incomplete idea of the 



^HC CHR interactions of the two electric fields which 



f'"'^' o-^c attract the positively charged hydrogen 



°""^\ ^/v- "«• • atom with different field strengths. 



CHR RHC^ j£^ £qj. 5^g£(- reasons, the heteropolar 



"••■^ "■"\ /'^° - groups (OH, COOH, CHO, NH^ etc.) of 



„ ^^ , , , , neighbouring molecules cannot come near 



Fig. 98. Hydrogen bonds be- ° ° 



tween polypeptide chains. enough together, their electric fields attract 



water molecules. Instead of hydrogen 

 bridges, a hydration layer is formed between them (Fig. 96, p. 145) and 

 it is obvious that with this kind of junction the cohesion depends on 

 the number of water molecules between the two end groups, i.e., on 

 their hydration. For this reason heteropolar cohesive bonds are j-^/zj-zV/Vf 

 fo hydration changes. 



Swelling depends largely on the presence of inorganic ions, in 

 which case the so-called ion series of Hofmeister holds good (see 

 HoBER, 1922). Their influence on swelling phenomena can be ex- 

 plained morphologically on the basis of the diameter and hydration 

 layers of the ions. Goldschmidt has calculated the diameters of the 

 ions from the distances between the atoms in the crystal lattice, and 

 the size of the hydration layers can be derived from the ion mobilities. 

 For the monovalent cations, for instance, the following radii have 

 been found (Table XVI). 



Obviously the small ions have thicker hydration layers than the 

 bigger ones. This is due to the fact that the water dipoles are attracted 

 more strongly as the distance between the centre of gravity and the 



