390 Scientific Proceedings, Royal Dublin Society. 



taining 300 cc. of water in each case contained very nearly three times the 

 amount of the dissolved gases contained in the tubes holding 100 cc. of water. 

 Obviously, if any concentration had taken place in the surface layers of the 

 columns of water, this should not have been observed. 



The rate of absorption of air by the water in these tubes proved, however, 

 to be so very slow that it was not possible to obtain a complete set of obser- 

 vations under constant conditions. The length of exposure varied from one 

 hour to 144 hours, and even after six days' exposure, complete saturation was 

 not reached. In addition, it was found that the variations of atmospheric 

 conditions during these long periods of exposure exercised a very consider- 

 able influence on the rate of solution, causing irregularities in the air-content 

 which could not be corrected for. 



In order to overcome this difficulty, it was thought advisable to provide 

 each tube with an enclosed atmosphere that could be kept at least at a 

 uniform temperature and uniform state of 'humidity during the course of 

 the experiments. For this purpose a modification of the constant-pressure 

 apparatus, that was used by one of the authors to measure the absorption of 

 oxygen by polluted waters, 1 was employed. 



The rate of absorption in these tubes was much the same as before, but 

 the conditions affecting the atmosphere above the water-surfaces were under 

 control. Thus some of the tubes were wholly immersed in the thermostat 

 to the level of the corks ; while others were only immersed to the same level 

 as the water surface inside them. In the one case, therefore, the air in the 

 tube was kept at the same temperature as the water, and evaporation from 

 the surface was reduced to a minimum ; while, in the other case, the air was 

 exposed to all the fluctuations of temperature of the laboratory, and con- 

 siderable evaporation and condensation took place. There was a marked 

 difference in the rate of absorption under these different conditions, it being 

 much greater in the tubes liable to fluctuation of temperature. 



In order to shorten the time during which it was necessary to keep the 

 tubes under observation, they were only half filled and laid on their sides, so 

 as to expose a greater area of water-surface — equal to the longitudinal section 

 of the tube — and a considerably reduced thickness of water to be saturated. 



The area of surface was in this way increased from about 12 square centi- 

 metres to about 120 square centimetres, and the thickness decreased to a 

 maximum of about 2 centimetres, and, further, the actual volume of water 

 used was only half the previous amount. Under these conditions the aera- 



1 Supplementary Volume VI, Fifth Report, Royal Commission on Sewage Disposal, 

 pp. 438-441. 



