34 



cules and their excited electrons with the crystal lattices of this 

 queer substance, water. 



This explains why there was no striking change in the behavior 

 of our fluorescent substances in glycerol on cooling; glycerol may 

 form a rigid mass at low temperatures but does not form crystals. 

 This explains also why an admixture of 10% glycerol or 2% 

 methanol to the water deprived it of its specific action on excita- 

 tion: because these admixtures destroyed the regularity of the 

 water crystals. 



When considering water structures we enter a fantastic and fas- 

 cinating world. Bridgman, in his studies on high pressure, could 

 distinguish between half a score of different ices. But we need not 

 go to ice to find structures in water. Bernal and Fowler, in their 

 classic paper showed water to have a quartz-like "crystalline" struc- 

 ture which is different from that of common ice which is tridymit- 

 like. Though Bernal's and Fowler's theory has somewhat been 

 modified by later researchers, its essential correctness has never 

 been questioned. In the HgO molecule the central O holds the two 

 protons on one side, which lends a strong dipole moment to the 

 molecule. The orbitals of the lone pair of electrons are directed 

 towards the other side, contributing to the dipole character 

 (Pople). The two protons can be shared, through H-bonds, with 

 two other water molecules, while the lone pair of electrons may 

 attract two protons from two other HsO's, so that each water mole- 

 cule can link up with four others. Since the four orbitals respon- 

 sible for these links point in nearly tetrahedral direction, a tetra- 

 hedral lattice will thus be formed. If water, in this state, is still 

 liquid this is due to the fact that the links can be broken, and, as 

 Pople emphasized, can also be bent easily. Above 0°C, heat agi- 

 tation does not allow the molecules to settle down to permanent 

 rigid lattice but all the same the tendency to form such a lattice 

 is there, keeping water in a "quasi crystalline" condition. Water 

 seems to have two melting points, one at 0°C, the point at which 

 the water goes from a rigid crystal, so to say, into a liquid one. 



