Molecular Constitution of Water. 485 



close to one another that the atoms of adjacent molecules 

 jostle one another almost as freely as do the atoms within a 

 single molecule. In the case of ice, by far the greater part 

 of its heat-energy consists of the kinetic energy of the 

 hydrogen and oxygen atoms, almost as if ice were a mech- 

 anical mixture of these, except that each oxygen atom and 

 its two combined atoms of hydrogen influence one another's 

 relative motions by the action of the chemical forces. In 

 the same way in the molecule of trihydrol (H 2 0) 3 , the 

 chemical forces must be regarded chiefly as controlling the 

 H 2 molecules into groups of three, thereby regulating their 

 arrangement, but not seriously affecting the motion of the 

 •constituent atoms in any other way. Let us us name the 

 oxygen atoms of (H 2 0) 3 A, B, C. Then for each there is 

 some point where the attractions of all other atoms for it are 

 in equilibrium ; call these for the three oxygen atoms a, (3, 7, 

 forming an equilateral triangle. Then on the average A will 

 vibrate through a in a direction perpendicular to (3y with an 

 amplitude id. The largest deformation of the equilateral 

 triangle commonly occurring will be when A is displaced id out- 

 wards and B and C w inwards, or what this configuration 

 changes to in half a period of its vibration when B and C are 

 displaced w outwards and A id inwards. Let the side of the 

 triangle be denoted by D, then the deformation of the 

 triangle would be conveniently measured by ic/D. When 

 the deformation reaches a certain value, the chemical equi- 

 librium of the three oxygen atoms becomes unstable, the 

 bonds AB AC may be said to break under the breaking-strain, 

 or the electrons which form the chemical bonds swing round 

 so that B and C are united by double bonds, and A is free 

 to assist in upsetting the equilibrium of a neighbour molecule 

 of (H 2 0) 3 , and so the process goes on. 



Now D and w must both be regarded as functions of / the 

 pressure and T the absolute temperature. Now the fact that 

 to melt ice at constant pressure we have to warm it up to the 

 melting-point, shows that w/D increases with T, as indeed 

 we should expect to be the case. Again, from thermo- 

 dynamics and experiment we know that if the pressure on 

 ice is increased by 1 atmo, the melting-point falls by "0075 

 •degree, and therefore w/D at constant temperature diminishes 

 with increasing pressure. Both properties of iu/D would be 

 accounted for by considering D relatively more affected by 

 pressure and le.-s by temperature than w. The dissociation 

 of some of the trihydrol in water into dihydrol by increase 

 both of temperature and pressure is explainable by these 

 same properties of 10 and D. 



