WATER, ITS PROPERTIES AND FUNCTIONS 233 



Although water does not chemically decompose salts dissolved in it, yet by 

 causing their dissociation into ions, it enables all kinds of reactions to take place 

 which do not occur between the solutes in their molecular state. It was shown 

 by Yeley (1910, p. 49) that pure nitric acid does not react with calcium carbonate. 

 The importance of ions in physiological processes has been abundantly illustrated 

 in the previous chapter and need not be further insisted on here. 



If now we look at a series of substances arranged in order of dielectric constants, heats 

 of vaporisation and conductivity for heat, we notice that there is an unmistakable con- 

 nection between these properties. It will also be found that these properties are related to 

 the critical pressures and to both the constants of van der Waals. So that, after all, some of 

 the wonderful properties of water are mutually dependent. 



THE CONSTITUTION OF WATER 



The actual percentage composition of water, as formed by two volumes of 

 hydrogen to one of oxygen, was proved by Cavendish (1781), although the true 

 explanation of the results obtained was not known until the experiments of 

 Lavoisier in 1783, as Cavendish held to the doctrine of phlogiston. 



It is only of recent years, however, and owing greatly to the influence of 

 Armstrong, that it has been realised that water cannot be correctly represented 

 by the symbol H 9 O, with the molecular weight of only 18, or rather it is only 

 under limited conditions that this can be done. 



In the first place, the freezing and boiling points are not at all where they 

 would be expected to be in a simple compound containing three molecules only of 

 gases with extremely low freezing and boiling points. In fact, comparing it with 

 similar compounds, as Jacques Duclaux points out (1912), the freezing point 

 should be about -150 and the boiling point 100. It appears then that the 

 molecular weight of water must be greater than 18; in other words, it must be 

 a polymerised or associated liquid, in which a number of molecules are united 

 together. Comparing formaldehyde, which is liquid at - 20, with its polymer 

 trioxymethylene, composed of three molecules of formaldehyde, we notice that the 

 latter is solid even at 150 ; so that considerable changes of properties occur even 

 when only three molecules are combined together, and although H 2 O ought to 

 boil at - 100, H 6 O 3 might well boil at + 100. 



We must suppose that chemical combination takes place between the simple 

 molecules when polymerisation takes place. Thus, although formaldehyde and 

 glucose have the same percentage composition, no one would regard them as the 

 same chemical substances. Also, at any given temperature, there is an equilibrium 

 between the polymers of water, which are mutually convertible, so that the 

 different chemical individuals are easily changed into one another, and the 

 chemical change is by no means so marked as in the example given above. 



We may now at once proceed to make use of the names proposed by Sutherland 

 (1900). The substance composed of single molecules, which does not appear to 

 exist as a liquid, is hydrol, that of two molecules is dihydrol, that of three 

 molecules is trihydrol, and so on. 



So far the theory is simple, but already several of the peculiar properties of 

 water can be explained by it. The degree of polymerisation, as a general rule, 

 increases as the temperature falls, so that cold water is not the same liquid 

 chemically as warm water and is less volatile; hence its vapour pressure falls 

 more rapidly than that of a simple liquid would. This is a favourable circumstance 

 in regard to the properties of water as a regulator of animal temperature, 

 since the cooling produced by its evaporation is greater the higher the tempera- 

 ture is. 



We saw that the specific heat of water is unusually high. Now when heat 

 is applied to water, it has to do three things : a part serves to heat the complex 

 molecules, another part to heat the simple molecules, and a third part to decompose 

 a certain number of complex molecules into simple ones. The specific heat of 

 water, furthermore, presents a minimum at about 30. The two first-mentioned 

 fractions of the heat probably increase regularly with the temperature, as is usual, 

 but the third rapidly decreases, being proportional to the concentration of complex 



