244 PRINCIPLES OF GENERAL PHYSIOLOGY 



Among these properties, we may note its high specific heat, its great latent 

 heats of solidification and of vaporisation, its good conduction of heat, which is 

 unusually high for a non-metal, its point of maximum density at 4 above its freez- 

 ing point, its high surface tension, its transparency to radiant energy, its solvent 

 powers, and, as a solvent, its chemical inertness is important, and its large dielectric 

 constant. In the majority of these, it stands higher than any other substance, and 

 where it is exceeded, it is only by one or two very exceptional liquids, such as 

 ammonia and prussic acid. While some of these are dependent on each other, 

 others appear to be independent. The manner in which each of these characteristics 

 intervenes in relation to living organisms is given in the text. 



Many of these properties find a satisfactory explanation in the nature of water 

 as a polymerised compound, consisting of three degrees of aggregates trihydrol, 

 a compound of three molecules of H 2 O, and apparently identical with ice ; dihydrol, 

 of two molecules, present in largest proportion in ordinary liquid water ; and finally 

 monohydrol, or steam, of single molecules. The relative proportion of these 

 to one another changes as the temperature varies, so that passing upwards the 

 concentration of the polymers decreases regularly. 



The application of heat, therefore, has to do three things : decompose the 

 polymers, heat the polymers and heat the single molecules ; the anomalies 

 connected with the specific heat of water are thus explained. 



The point of maximum density at 4 can be explained on the assumption 

 that ice, or trihydrol, exists in liquid water, since ice at has a lower density 

 than water at O c ; and, as water is heated from upwards, there are two opposite 

 processes going on, dilatation of the molecules, according to rule, and contraction, 

 due to change of ice to water. Since the latter process is preponderant at the 

 lower temperatures and nearly absent at the higher, there must be a point where 

 the difference between them is least. 



The unusual increase both of viscosity and of compressibility as the temperature 

 falls is also explained by the existence of polymers. 



Certain other properties of water are to be explained by the fact of its being, 

 of all liquids, that one containing the greatest number of molecules per unit 

 volume. 



The proof of the existence of single molecules, monohydrol, in liquid water 

 is given by consideration of solution volumes and will be found in the text. 



Many solutes, both electrolytes, ions and non-electrolytes, take up a certain 

 number of molecules of water, forming "hydrates." Whether this is to be 

 regarded as chemical combination appears to be rather a matter of opinion. 



It is shown that the theory of osmotic pressure, as given in Chapter VI., is 

 not affected by the hydration of solutes nor by the polymerisation of solvent. 



Water, to a very small extent, is electrolytically dissociated. The value of 

 the dissociation constant, obtained by four independent methods, is practically 

 identical, a satisfactory proof of the correctness of the assumption. 



This electrolytic dissociation of water is the cause of the " hydrolytic dissocia- 

 tion " of salts of weak acids and bases dissolved therein. 



The properties of water as a catalyst are, in many cases, of importance. 



The concentration of water in reversible reactions of hydrolysis and synthesis 

 is a potent factor in the regulation of reactions in protoplasmic systems. There 

 are, no doubt, mechanisms of a colloidal nature present in such systems and 

 effective in bringing about changes in the active concentration of water. Diminu- 

 tion of water favours synthesis, increase favours hydrolysis. 



There is evidence that certain bacteria can be completely deprived of water 

 without causing their death. When organisms become encysted, it appears that 

 they do not become completely dried, but that the membrane of the cyst is 

 practically impermeable to water. 



