20 Scientific Proceedings, Royal Dublin Society. 



with solutions of sodium chloride, having a wide range of concentration, to ascer- 

 tain the effect of concentration of the sodium chloride on the rate of aeration. 



Experimental. 



The de-aeration of the water employed in these experiments was effected by 

 distillation in vacuo. In order to obtain water quite free from atmospheric gases, 

 the distillation was at first carried on in a very slow current of liydrogen, or of 

 carbon dioxide; but, although the resulting water was air-free, it was always fo\iiid 

 to contain undesirable traces of hydrogen or of carbon dioxide, according to which 

 gas was employed during the distillation. It was consequently decided to rely 

 upon distillation, under the reduced pressure obtained with the aid of a good water 

 vacuum pump, employing a slow current of air, filtered througli glass wool, to 

 overcome difficulties from "bumping." The nitrogen content of the de-aerated 

 water obtained did not exceed 1 c.c. per litre, which, in the case of pure water, 

 amounts to about 7 per cent, of saturation at 15° ( '. 



It was found unnecessary to determine the nitrogen content of each sample of 

 water at the commencement of each aeration experiment, as it was found to be 

 practically constant immediately after the de-aeration. 



The glass tubes used in the experiments were about 3 cms. in diameter, and 

 varied from 9 to 12 feet in length. T-pieces were fused on, at regular intervals, 

 along the lengtli of the tube, for drawing off samples of the water columns at 

 different levels, and the lower ends of the tubes were sealed. Tlie upper end of 

 the tube was closed by a rubber stopper, fitted with an inlet and outlet tube, by 

 means of which connexions were made to the pump and distilling apparatus ; 

 so that the water could be distilled directly into the experimental tube when 

 desired. 



In the case of salt solutions, tlie required quantity of sodium chloride was 

 placed in a five-litre flask, connected with the vacuum distilling apparatus by 

 means of a thick-wail rubber tube, and the required volume of pure water was 

 allowed to distil into the flask and dissolve the salt, the exact concentration of 

 the solution being subsequently determined by analysis. A screw clip was then 

 used to close the connexion with the pump, the flask detached with its rubber 

 connecting tube, and attached, witli the aid of the same connecting tube, to tlie 

 experimental tube. When the free end of this connecting tube was completely 

 filled with freshly distilled water, it could be attached air-free to the previously 

 exhausted experimental tube. After attaching the flask to the tube, the latter 

 could be filled by inverting the fiask and unscrewing the clip, when tlie solution 

 flowed into the tube. 



Experimental tubes were also employed, which were furnished with quill 

 capillary tubes, instead of side tubes, for drawing off samples. These tubes 

 passed upwards through a rubber stopper at the bottom of the experimental tube 

 to different levels inside, the external ends being attached each to a piece of 

 thick-walled rubber tubing closed by a screw clip. 



Eacli experimental tube was attached to a stout lath of wood for facility of 

 removal from one laboratory to another. The room in which the aeration was carried 

 out was one which received no direct sunshine nor heat from artificial sources, so 

 that the temperature prevailing in it was not subject to sudden fluctuations. In the 

 earlier experiments, a zinc cylinder, 14 inches in diameter, and filled with water, 

 was used as a jacket for the tubes. But it was subsequently found that a good 

 wrapping of asbestos cloth was a sufficient guard against sudden variation in 



