164 The Realm of Nature CHAP. 



proportion of calcium or magnesium carbonate than near 

 the surface. It is remarkable that more than three-quarters 

 of the whole is made up of common salt, while the propor- 

 tions of carbonates and of silica are very minute. Silica in 

 carefully filtered sea- water never appears to exceed I part in 

 250,000 or 0-0004 P er cent. The proportion of sulphates is 

 nearly the same as in the salts of river-water. Some geo- 

 logists suppose that the sea consists merely of concentrated 

 river-water ; and even on the more probable assumption 

 that sea-water contained salts in solution derived from the 

 primeval atmosphere, it is evident that some agent must 

 be at work withdrawing silica and carbonates from river- 

 water as it enters the sea. That agency is known to be 

 the power of living creatures plants and animals to make 

 themselves shells or skeletons of silica or of calcium carbon- 

 ate secreted from the water ( 273). Sea-water is slightly 

 alkaline, probably on account of its containing bicarbonates 

 in solution. It dissolves carbonate of lime, especially when 

 subjected to great pressure. 



223. Salinity. The salinity of sea-water is the amount 

 of dissolved salts contained in 100 parts. One hundred 

 pounds of average sea- water contain about 3-5 pounds of 

 dissolved salts, and thus the average salinity is said to be 

 3-5 per cent. It is difficult to measure salinity directly, as 

 some of the salts decompose when the water is boiled down. 

 The density of sea-water, however, depends on its tempera- 

 ture and on the salinity, so that if the density is always 

 measured at the same standard temperature, or corrected 

 to it, the differences of density are due to differences of 

 salinity alone. For example, if a bottle contains exactly 

 1000 grains of pure water at the temperature of 60 F. it 

 would contain 1013 grains of sea-water which held 1-75 

 per cent of salts in solution, and 1026 grains of water 

 holding 3-5 per cent of salts. Density (specific gravity) is 

 measured most easily by means of a delicate hydrometer, 

 but most accurately by weighing a carefully measured por- 

 tion of the water. The standard temperature to which 

 density of sea- water is calculated is usually 32 F. or 60 

 F. in English-speaking countries, and o C. or 17-5 C. on 



