Grafts et al. — 16 — Water in Plants 



% the space occupied by the molecule since it requires the same energy 

 to make a hole of molecular size against the forces of cohesion as to 

 vaporize a molecule. A property peculiar to liquids is that such holes are 

 shared communally by all molecules. For a thermodynamic consideration 

 of viscous flow of electrolytic solutions see Harned and Owen (1943, 

 p. 67). 



In their theory of the viscosity of liquids as a function of temperature 

 and pressure, Ewell and Eyring (1937) state that the unusually high 

 viscosity of water is brought about by the hydrogen bond structure. When 

 a molecule in such a liquid flows, it must not only break van der Waals 

 and dipole forces, but hydrogen bonds as well. As water is warmed the 

 number of these hydrogen bonds that must be broken decreases rapidly 

 and this is reflected in a decreasing coordination number. Assuming that 

 the activation energy for cavity formation in the liquid is Ye the molec- 

 ular vaporization energy, they calculate the following coordination num- 

 bers for water : 



Stewart (1939), on the other hand, considering the mean coordination 

 of large numbers of molecules, finds that the coordination of water in- 

 creases with the shift of structure from the hexagonal, four coordinate lat- 

 tice of ice to the more closely packed structure of water. He emphasizes 

 the extremely labile character of the liquid and points out that the presence 

 of ions or rising temperature does not contract the existing structure of the 

 fluid but tends to shift the coordination and increase the closeness of pack- 

 ing. The views of Stewart and the previously cited workers may be 

 brought into agreement if one considers the difference between the actual 

 coordination of a single molecule at a particular moment and the average 

 coordination of a large group over an indefinite time. With increasing 

 temperature the number of actual bonds per molecule at a given time will 

 decrease ; on the other hand, because of the increasing thermal agitation the 

 possibilities for contacts between different molecules increase immensely 

 and the tendency toward bonding is greater. This is particularly true dur- 

 ing the shift from the stable structure of ice to the labile structure of liquid 

 water ; the increased closeness of packing still furthers this tendency. In a 

 later paper, Stewart (1943) states that increase in temperature (0° to 

 4° C.) breaks hydrogen bonds and alters the structure of water, decreasing 

 its molal volume. Ions in aqueous solution break hydrogen bonds and alter 

 water structure, decreasing its molal volume ; they consequently increase 

 the pure temperature expansion of the solvent. He uses the phrase 

 "smeared out" to describe the relation of liquid to crystal structure of 

 water (1944). He concludes that water should be visualized as having 

 open tetrahedral structure and that the true aqueous solution develops a 

 new liquid structure in which both solvent and solute participate. Finbak 

 and Viervoll (1943) picture the structure of water as a three-dimensional 

 network of pliable branched chains of tetrahedrons with the corners linked 

 together in such a way that rotations about the lines between centers of 

 neighboring tetrahedrons may occur. 



Eley (1944) points out that for water the energy required to produce 

 a cavity is small at low temperature and increases until it reaches 80° C, 



