380 Scientific Proceedings, Royal Dublin Society. 



follows that as river-water rising in a cold region in contact with limestone 

 flows to a warmer region it must deposit calcium carbonate and become more 

 alkaline in the process, since as temperature rises the carbon dioxide content 

 of the water is reduced, so that the value pH 8'37 at 16° 0. is decreased at 

 0° C. and increased at 35° C. The hydrogen ion concentration of pure water 

 varies, however, in the opposite sense. The water of the Biver G-andak at 

 Pusa, in Bihar, India, was found to be at about pH 8'6, the air tempera- 

 ture being about 25° 0. in November, falling from a much higher value in 

 October. It may be remarked that sea-water is at about pH 8'2 in the 

 Atlantic near the British Isles, but shows seasonal variations, due to photo- 

 synthetic action of plankton. The soil in many parts of Bihar, in the basin 

 of the Ganges and its tributaries, is a fine silt, containing 10 to 40 per cent, 

 of calcium carbonate. This soil, with remarkable uniformity, gives pH 9'0, 

 or thereabouts, when freshly shaken up with water free from dissolved salt-s 

 and cai-bouic acid. On standing, however, the pH value decreases owing to 

 bacterial action. This deposition of calcium carbonate with the silt appears 

 to stand in relation to the temperature changes undergone by the water in 

 its progress from the cold mountain regions through the plains. 



(b) Magnesium salts. — In the foregoing, calcium carbonate has been 

 considered as the sole source of alkalinity. Many natural waters, however, 

 contain magnesium carbonate. It has been shown by Moore, Prideaux, and 

 Herdman (1915) and by M'Clendon (1917) that the major portion of the 

 alkalinity of the sea is due to magnesium salts, on account of the greater 

 solubility of magnesium carbonate, about 0100 grams per litre, as against 

 0'0131 grams for calcium carbonate. Moore states that sea-water may 

 become more alkaline than pH 91 owing to the removal of carbon dioxide 

 by algae. The writer has found that it may even reach pH 9-7, viz., pH 9-95 

 with thymol phthalein uncorrected for salt error. 



Corresponding with the greater solubility, magnesium carbonate, through 

 hydrolysis, gives rise to a greater concentration of hydroxyl ions. Thus with 

 cresol phthalein and thymol blue it was seen that a saturated solution was 

 at least as alkaline as pH 9-6. Using as indicator thymol phthalein an 

 exact match was obtained at pH 10-0, though this indicator is by no means 

 at the end of its range at this point. From this it may be seen that neither 

 soil extract nor natural water can surpass pH lO'O owing to alkalinity 

 derived from magnesium carbonate. Such a pH value would, however, be 

 fatal to most, if not all, plants. Since calcium carbonate cannot give a value 

 above pH 9'01, it is evident that higher alkalinity is possible where 

 magnesium salts are present with calcium carbonate, as in sea-water. 



