392 



J. D. BERNAL 



sufficient to cover all terrestrial life, we can set out a sequence of five inter- 

 particulatc forces or mutual interaction energies. In this sequence each of these 

 corresponds to a longer range of action and a weaker interaction energ}' than 

 those preceding it and, what is perhaps most important, the steps though broad 

 are separated by wider gaps. 



(a) (b) (c) (d) (e) 



Fig. 6. Types of diagrammatic aggregation of helical molecules. 



(a) — simple helix; (fc) — interleaved helices on adjacent axes; (c) — interleaved helices 



sharing common axes (double helix); (d) — coiled coil helix (after Pauling); 



(e) — combination of coiled coil helices to form twined cables. 



Only homopolar forces, the hydrogen bond and ionic forces are reasonably 

 well understood and will not be further discussed here (see Table i). The 

 mechanism of the remaining two, which I have called cryohydric and long-range 

 forces, are still unexplained theoretically or are the subject of current controversy. 

 There is, however, little doubt about the reality of these forces and of their range 

 of action because these are revealed by determinate structures which so far can 

 only be explained by postulating them. They are in my own view distinct. The 

 cryohydric forces imply an ordered but not necessarily ice-like grouping of water 

 molecules such as, for instance, occurs in the rare-gas hydrates. They are opera- 

 tive up to the range at which such structures can be maintained, itself a function 

 of the temperature, at room temperature something of the order of 20 Â. They 

 are also a function of the size of the particles they bind together, being greater 

 the larger and flatter these are. In the case of montmorillonite clay, where these 

 conditions are optimal, the range is at least up to 40 A [9]. 



The long-range forces on the other hand operate through a medium indis- 

 tinguishable from water or ionic solution, incapable of showing rigidity. Whether 

 they are due to simple Van der Waals forces, demonstrated to exist between 

 extended solids [10, 11], or, as Kirkwood has suggested [12], due to virtual 

 ionization, they seem to operate wherever there are particles of over 100 Â in 

 diameter containing ionizable groups. In terms of energy there seems to be a 

 minimum which leads to a condition of equilibrium at a definite distance, pro- 

 vided that the thermal energy is not greater than that of the interaction minima. 

 The position of the minimum is not notably sensitive to temperature but as old 

 experiments on tobacco mosaic virus [13] have shown, it is much affected by pH. 



