Crafts et al. 



12 



Water in Plants 



earlier interpretations of such studies. Ice apparently has lattice structure 

 but workers disagree on the details. Pennycuick accepts the interpreta- 

 tion of Bragg that each oxygen atom is surrounded by four atoms of hy- 

 drogen and that each hydrogen atom lies symmetrically between two oxy- 

 gen atoms. Assuming a tetrahedral structure for the water molecule and 

 a hexagonal ring structure for association of water molecules, a lattice 

 structure may be visualized. 



Bernal and Fowler (1933) discuss in detail the structure of ice and 

 water from the standpoint of infra-red and Raman absorption spectra and 

 x-ray diffraction. They see no reason to assume that the molecules of 

 liquid water differ from those of steam except for small mutual deforma- 

 tions. They picture the electronic distribution in the simple water molecule 

 as shown in Figure 6. 



Fig. 7. — The crystal lattice of ice as pictured by Bernal and Fowler (1933). 



This shows the oxygen atom at the center with two hydrogens in a hori- 

 zontal plane with the HOH angle 103°-106°, very near the tetrahedral 

 angle. The two charges Px represent the negative charges in the vertical 

 plane at right angles to the HOH plane. The net electronic density distri- 

 bution resembles a tetrahedron with two corners of positive and two of 

 negative charge. The radius of the molecule is taken to be 1.4A. 



Analysing the x-ray scattering curve for liquid water, Bernal and 

 Fowler arrive at a fundamental grouping of five molecules, one in the 

 center surrounded by four others to form a tetrahedron. This structure, 

 however, is not flexible enough to cover all requirements imposed by experi- 

 mental results. Studying the distribution functions for the neighbors around 

 a water molecule they conclude that water may have three chief forms 

 of arrangement : water I, tridymite, ice-like, present to a degree below 4° C. ; 

 water II, quartz-like, predominating at ordinary temperatures ; water III, 

 close packed, ideal liquid, ammonia-like, predominating at high tempera- 

 tures below the critical point at 374° C. Close packed in the above sense 

 refers to a uniformity of distribution in which deviations from the average 

 spacing are at a minimum. These forms pass continuously into each other 

 with change in temperature. The liquid is homogeneous at all tempera- 

 tures but the average mutual arrangements of the molecules resemble 

 water I, II, and /// in more or less degree. 



