22 FRESH-WATER BIOLOGY 



properties. If water contracted on freezing ice formed at the- sur- 

 face would sink to the bottom, more would be formed and accu- 

 mulate at the bottom in winter. Here it would thaw very slowly 

 or not at all in summer and the entire surface of the earth would 

 thus quickly become refrigerated. The expansion of water on heat- 

 ing is also very important as it is responsible for the setting up of 

 currents which ventilate the aquatic environment. 



Water is by far the most general solvent for other substances. 

 No other liquid will dissolve so many common substances. Though 

 it is one of the most stable and inert compounds, like salts in solu- 

 tion, it dissociates into parts or ions and a very small proportion 

 of pure water is in the form of H + (the cation bearing a positive 

 electric charge) and OH~ (the anion bearing a negative electric 

 charge). These ions are known respectively as hydrogen and 

 hydroxyl ions. At 25 C. 1000 grams of pure water contain 

 0.000,000,1 gram of ionized hydrogen and 0.000,001,7 gram of ionized 

 hydroxyl. Salts in solution in water are ionized. For example 

 common salt, NaCl, exists chiefly as Na + and Cl~. Henderson 

 states that solutions in water are the best source of ions. The 

 variety and complexity of the environment of aquatic organisms 

 and the number and variety of chemical reactions are increased by 

 the presence of ions. 



As compared with air, water is much denser, being 773 times as 

 heavy. Gases and other solutes are presented to organisms in 

 solution and gases need not be taken into solution by surfaces 

 moistened by body fluids as in the case of land organisms. The 

 diffusion of gases is less rapid in water than in air. Some food 

 substances are in solution in water; many food organisms float in 

 it on account of its great density. This enables some aquatic 

 animals to rest in one position and secure food without effort. 



PHYSICAL AND CHEMICAL CONDITIONS 



Physical conditions can be separated from chemical conditions 

 only arbitrarily. Combinations of the various physical conditions 

 in water may be included under the term physiography. Physi- 

 ography in the broad sense includes topography of the land asso- 



