40 HYDROLOGY 



contact with in their course. These substances may be either 

 solids, liquids or gases. The quantity of a solid which may be 

 dissolved by a liquid is fixed and limited, and is always the same 

 for the same temperature, the solubility however, generally 

 increasing with the temperature. The same quantity of gas will 

 also be dissolved by a liquid if the temperature and the pressure 

 remains the same, the volume of gas dissolved being proportional 

 to the atmospheric pressure. Rain water always contains in 

 solution a certain amount of the natural gases of the atmosphere. 

 These are, however, not dissolved in proportion to their occur- 

 rence in the atmosphere, but more nearly to the solubility of the 

 gases. Deep waters and waters of springs which have been under 

 pressure carry in solution larger percentages of carbonic acid 

 gas than natural waters. 



There is a distinct difference between substances in solution 

 and in suspension. When in suspension the substance still retains 

 its physical identity, although it may be held in an exceedingly 

 finely divided state and thus be carried in suspension for indefinite 

 periods. When the water is at rest the heavier suspended par- 

 ticles are soon deposited. 



Volume. For all practical purposes water may be considered 

 non-compressible. The coefficient of compressibility ranges from 

 0.00004 to 0.00005 per atmosphere at ordinary temperature the 

 coefficient decreasing as the temperature increases. 



Table XVII gives the relative volume and weight of pure 

 water at various temperatures, as compared with its volume at 

 39.2 F. 



Critical Temperatures. There are four temperatures of water 

 which are often used in physical calculations and which should be 

 kept in mind, viz.: 32 F. or C., at which pure water freezes at 

 one atmosphere pressure (sea level). The weight of ice is 57.5 

 pounds per cubic foot, and when floating in pure water 92 per cent 

 of its mass is submerged, while in sea water about 89 per cent. 



39.2 F. or 4 C., which is the approximate point of maximum 

 density of pure water. 



62 F. or 16.67 C., which is the British Standard tempera- 

 ture, and which is used as a basis in calculating the specific gravity 

 of bodies in England and United States. 



212 F. or 100 C. is the boiling point of pure water at atmos- 

 pheric pressure. 



