324 Prof. Adeney and Mr. Becker: Determination of Rate of 



This modification in the method of employing the tubes- 

 brought the series of observations under better control, and 

 more concordant results were obtained ; nevertheless the 

 observations showed some irregularity, which, though small, 

 could not be regarded as negligible. Attempts were there- 

 fore made to work out an entirely new method of experi- 

 menting, and to devise some method by which the water 

 could be uniformly mixed during aeration, and still preserve 

 an unbroken water-air surface. 



One of the simplest means of effecting this seemed at first 

 to be to pass a known volume of air, in a slow and uniform 

 stream of bubbles, through a given thickness of water, in an 

 observed time, and to determine the amount of air dissolved 

 during that time. Experiments carried out in this way 

 yielded results which afforded a remarkably uniform curve; 

 but attempts to gain an accurate estimate of the water-air 

 surface exposed proved unsuccessful, owing to the variation 

 in the size and shape of the bubbles, as well as to the- 

 irregularity of the paths they took in ascending the water. 



With a view to bringing the method under better control,, 

 efforts were made to direct the course of the bubbles into a 

 straight line, and it was found that the conditions prevailing,, 

 when a large bubble is allowed to ascend a narrow tube filled 

 with water, were almost exactly those sought. In this 

 case, the surface tension keeps the water-air surface unbroken 

 during its passage up the tube, and since the bubble occupies 

 nearly the whole cross-section of the tube, the water is 

 exposed to the air in an extremely thin layer ; while the 

 upward movement of the bubble produces a very perfect local 

 mixing of the water. 



Since the bubble tends to assume a simple geometrical 

 form, the calculation of the area exposed, from measurements,, 

 is rendered comparatively simple, provided that the mea- 

 surement of length is made when the bubble is in motion. 

 In practice this was achieved by taking an instantaneous 

 photograph of the bubble with a glass metre scale placed 

 alongside of it, in the same focal plane. 



Some difficulty was at first experienced in getting the 

 edges of the meniscus well defined ; but this difficulty was 

 finally surmounted by colouring the water slightly with 

 eosin, and taking the photograph by transmitted light from a 

 powerful arc-lamp. 



After some trials, it was found that the simplest method of 

 measuring the amount of absorption of air, after each 

 excursion of the bubble, was by measuring the loss of 

 pressure exerted by the bubble. 



