WATER 97 



ing a very large quantity of water must re- 

 main liquid ; a body of water at 0° centi- 

 grade can warm up a very large amount of 

 colder air with the formation of a very small 

 quantity of ice. Thus the permanency of 

 the ocean, and the moderating effect of water 

 upon cold climates are very nearly maximal. 

 These are also facts, directly dependent upon 

 the physico-chemical nature of water, which 

 are remarkably favorable to the organism. 



Still more important is the latent heat of 

 evaporation of water. Wherever water is 

 in contact with the air, evaporation must take 

 place until, if the system be of small dimen- 

 sions, equilibrium is established between aque- 

 ous vapor and the liquid; in short until the 

 air is saturated with water. Unlike freezing, 

 which occurs only at one particular tempera- 

 ture, this process goes on continuously through- 

 out all ranges of temperature at which liquid 

 water can exist, and even upon ice at low tem- 

 peratures. It is always accompanied by the 

 conversion of heat, in the' amount measured 

 by the latent heat of evaporation, into other 

 forms of energy ; the heat becomes latent. 

 And since air in contact with water is rarely 

 saturated with aqueous vapor, owing to the 

 constant movement of the atmosphere, the 

 process of evaporation, with the accompany- 



