410 ScienUfic Proceedings^ Royal Dublin Society. 



less vigorous photosynthesis of the diminished plankton and water plants 

 also tend to increase the amount of carbon dioxide ; and, furthermore, the 

 lower temperature of the water results in a large amount being retained in 

 solution. These factors also tend to diminish the pH values, namely, to 

 increase the hydrogen ion concentration. 



From what has been said already it will at once be recognized that these 

 results recorded in Table III point to the effect of the carbon dioxide from 

 the air and from the soil gases in lowering the reaction that could be pro- 

 duced by the amount of calcium carbonate present. Even with upland water 

 from Dartmoor taken from the li. Yealm at Blachford the acidity pH 6'8 is 

 entirely due to excess of carbonic acid. On driving this off, by boiling in a 

 carefully tested bard glass tube, as a maximum value pH 8'5 was reached. 

 Assuming this to be due to calcium carbonate (viz., not to magnesium), it is 

 pos.sible to calculate the concentration of this salt. A saturated solution eon- 

 tains O'Olol gram per litre, and gives as reaction pH 9'01.5 or Oo„ 1'05 x 10"°. 

 Now, pH 8-5 corresponds to Cqh 3-2 x 10"' ; and therefore to 0-0040 

 gram per litre if it be assumed that at these dilutions the concentrations 

 of the hydroxyl ions are proportional to the amounts of calcium carbonate. 



The assumption may not be quite correct ; for a more concentrated 

 solution it certainly would be erroneous (see Clark, 1920, p. 29). To test 

 this directly, a solution was prepared by boiling distilled water with calcite. 

 In reality, the solution was not quite saturated, as it gave pH 8'96 instead 

 of pH 9'01. This was diluted with the same distilled water, which when 

 freshly boiled was at pH 7T, as shown by brom thymol blue and phenol 

 red, two volumes being added to one of calcite solution. When freshly boiled, 

 this mixture was at pH 8'6 to both cresol red and thymol blue. Since it 

 contains 0'004 gram per litre, the calculation showing that the solution at 

 pH 8'5 contained 0"0040 gram must be tolerably correct. This method for 

 finding the bicarbonate content of such soft waters is very rapid ; its use will 

 be treated in more detail elsewhere. The solution at pH 8'5 is of normality 

 4 X 10"^ with respect to calcium carbonate. This is just over three-tenths 

 of the amount required for saturation in absence of carbon dioxide, but only 

 about six per cent, of saturation when in equilibrium with the atmosphere. 

 Photosynthesis in Blachford water cannot therefore produce a reaction more 

 alkaline than pH S'o as an upper limit. This was tested with a small glass 

 aquarium which had stood nearly four months with Blachford water and the 

 plants growing naturally in it. In subdued light the tank was at pH 6-8. 

 After an hour and a quarter in full October sunshine it had risen to pH 7'6, 

 and after three and a half hours to pH 8'6. This is slightly above the limit 

 reached by boiling the Blachford water direct, but the prolonged exposure 



