66 Hydrogen-ion Concentration and Electrical Conductivity 



ature rises, the carbon dioxide is driven out of the water and the 

 alkahnity increases; whereas, if the temperature falls, the water 

 attains an increased capacity for absorbing CO2 from the atmosphere 

 or of retaining CO2 derived from animals or plants, and the H-ion in- 

 creases in the water. Thus, in our observations of Ph off theAtlantic 

 coast of North America, the cold water off Yarmouth, Nova Scotia, 

 which was 1.4° C, had 7.96 Ph on March 26, while the tropical water 

 of the Sargasso Sea had 8.25 Ph at 23.55° C. on March 10, 1918. 



In lagoons such as that of Tortugas, Florida, and inclosed shallow 

 areas, McClendon found there was a diurnal variation in the Ph, the 

 water becoming more alkaline by day and relatively acid during the 

 night. This he correctly attributed to the effect of photosynthesis by 

 plant life, which is active in daylight but ceases during the night. 

 Over the deep sea this effect is apparently neutralized by the stirring of 

 the water due to waves, and perhaps by a moderate amount of inter- 

 change between the surface water and the deeper layers. Over shallow 

 regions, where the water may become impounded in tide-pools at low 

 tide, the effect of photosynthesis is often very marked, the Ph chang- 

 ing greatly while the temperature may change but little. Thus, over 

 the Aua reef-flat at Pago Pago Harbor, Samoa, where the water was 

 impounded and stagnant at low tide of the spring tide of July 16, 

 1920, the conditions as compared with those of the ocean-water just 

 seaward of the outer edge of the reef-flat were as shown in table 2. 



Table 2. 



At 27° C. the sea-water would be saturated with oxygen if it con- 

 tained 6 c. c. of oxygen per liter of water, but we see that the ocean- 

 water outside the reef contained only 4.67 c. c. of oxygen and was 

 therefore below saturation, while the shallow water of the reef-flat 

 which had been impounded in sunshine for about 2 hours at low tide 

 had 8.44 c. c. of oxygen per liter, thus having gained 3.77 c. c. of 

 this gas per liter in this short time, and becoming supersaturated by 

 about 2.4 c. c. of oxygen per liter. The rise in Ph from 8.25 to 8.6 was 

 of course due to the loss of CO2 resulting from the photosynthesis of 



