CHAPTER VIII 

 WATER, ITS PROPERTIES AND FUNCTIONS 



THERE is no doubt that if water were as uncommon a liquid as, say, amyl-alcohol 

 or toluene, it would be looked upon as endowed with the most wonderful properties. 

 Common as it is, ancient philosophers like Thales regarded it as the origin of all 

 things, and the development of science has shown how important it is in all the 

 phenomena with which we have to deal. It is chosen to fix standards of density, 

 of heat capacity and so on ; most of the reactions with which chemistry is 

 concerned take place in aqueous solutions. The action of water, in its several 

 forms of ice, liquid or vapour, is the chief factor in geological changes. Finally, 

 all physiological actions have their seat in systems containing water as an essential 

 component. 



We have already had occasion to take some account of its intervention in 

 protoplasmic activity, in the production of the colloidal state, in permeability and 

 osmotic pressure, and, in the previous chapter, in the dissociation of electrolytes. 

 We turn now to consider its various physical and chemical properties in turn, 

 together with their importance in vital processes. For many points to which 

 attention is directed, I may acknowledge my indebtedness to the third chapter 

 of L. J. Henderson's "Fitness of the Environment" (1913), to which the reader 

 is referred for more details. 



HEAT CAPACITY 



Of all solids and liquids under ordinary conditions of temperature and pressure, 

 water has the highest heat capacity, or specific heat. In other words, it takes 

 more heat to raise the temperature of a given mass of water by a given amount, 

 than it does in the case of any other of these substances. Liquid water is therefore 

 chosen as the unit of specific heat, and in consequence also to define the unit of 

 quantity of heat. The small calorie is that amount of heat required to raise the 

 temperature of one gram of water from to 1 C. 



The law of Dulong and Petit, that the specific heat of an element varies 

 inversely as its atomic weight, shows that a substance to have a high heat 

 capacity must consist of elements whose average atomic weight is low. Compounds 

 of hydrogen obviously will have the first place. 



The most general way in which this fact of the high specific heat of water is 

 important to life is the tendency of the sea, lakes, and rivers to prevent any 

 considerable change of temperature. It also enables vast quantities of heat 

 to be transported from the hotter to the colder parts of the earth by means of 

 ocean currents. Naturally, other properties of water, such as latent heat of 

 evaporation, etc., play a large part in maintaining a constant temperature. 



The high specific heat of water is directly favourable to the living organism, 

 composed as it is, in its active parts, of some 80 per cent, of water. The heat 

 produced by muscular activity would otherwise cause a great rise in the temperature 

 of the body before it could be eliminated from the surface by radiation and 

 evaporation. The more highly organised a creature is, the more sensitive are 

 the delicate adjustments of its chemical and physical processes to slight changes 

 in temperature. 



As L. J. Henderson points out (p. 91), the most striking change in modern laboratories 



