Adenry, Lkonakd, and Richardson — Aeration of Water. 21 



temperafcure. Manipulative difficulties %vei-e thereby considerably reduced, and 

 it was possible to work with a greater number of tubes simultaneously. 



A recording thermometer was kept in the room, and the greatest variation in 

 temperature in the room, during the exposure of any one tube, did not exceed 4-5 

 degrees centigrade. This variation, however, was exceedingly slow — too slow to 

 give rise to convection currents. 



The air passing over tlie surface of the water in the tubes was first dried by 

 calcium chloride ; and. the inlet tube was so arranged that the air did not play 

 directly on to the surface. The inlet and outlet tubes were fixed at a distance of 

 1-5 to 2 inches aboA'e the water. The moist air issuing from the tube was passed 

 through weighed CaCl, tubes, so that the amount of water evaporated could be 

 determined for each tuije. In most of the experiments three tubes were connected 

 in series, drying tubes being placed so that the moist air from one was dried 

 before entering the succeeding tube. 



A filter pump, worked by a constant head of water, seven feet in height, was 

 at first employed to aspirate the current of air through the tubes, but subsequently, 

 owing to shortage of the town water supply, caused by the drought last summer, 

 a different arrangement had to be substituted. An electrically driven small air 

 pump was utilized to force air into a large glass vessel, which acted as an equalizer, 

 and thence through the drying vessels and experimental tubes. This arrangement 

 worked very satisfactorily. 



The experimental tubes, having been filled with de-aerated water, were fixed 

 in a vertical position; the stoppers, which had been used during filling, were 

 removed, and were replaced by others carrying inlet and outlet tubes for the air 

 current. The air current was continued for two or three weeks in the case of salt 

 solutions, and three to eight weeks for pure water. The apparatus employed for 

 the determination of the dissolved gases was of the form devised by one of the 

 authors.^ 



Method of withdravjing Samples frovi the Experimental Tuhes. 



Samples of water were withdrawn from an experimental tube, without at 

 any time allowing them to come into contact with the air, with the aid of a 

 modification of the gas burette used for the analysis of the dissolved gases. 



By lowering the mercury reservoir, attached to the burette, a known volume 

 of water was drawn from the tube into the latter, and thence transferred to a 

 Plimpton gas holder to await examination. The depth from which the sample was 

 drawn and the temperature of the water were at the same time noted. 50 c.c. 

 of water were usually taken for the extraction and analysis of the dissolved 

 gases in the case of the surface layer; and 100 c.c. for samples drawn from lower 

 levels. 



Determination of Saturation Values for Nitrogen of Sodium Chloride Solutions. 



In order to calculate in percentages of saturation the observed rates of solution 

 of atmospheric nitrogen by solutions of sodium chloride of the various concentra- 

 tions employed in this investigation, it was necessary to determine the saturation 

 values for atmospheric nitrogen of each solution. This was done by filling large 

 tubes, about 5 cms. in diameter and .30 cms. in length, about two-thirds full of 



• Sci. Trans. R.D.S., vol. v, Series 1], p. 548 ; also Supplemental vol. vi. Fifth Report of 

 the Royal Commission on Sewage Disposal, p. 99. 



f2 



