394 Scientific Proceedings, Royal Dublin Society. 



The length of the bubble while in motion was measured by taking a 

 number of instantaneous photographs of it while it travelled up the tube. 

 A millimetre scale engraved on glass was placed in the same focal plane as 

 the centre of the tube, and an image of this scale appeared alongside the 

 bubble on the plate. 



On comparing this scale with a standard millimetre scale, it was found to 

 be considerably in error, and means were adopted to correct for this. 



By the use of a micrometer eyepiece in the microscope the length of the 

 bubble was read off on the scale photographed on the plate. The two points 

 corresponding to these readings were found on the actual scale, and the 

 distance between them read by means of the standard millimetre scale and 

 the micrometer. In this way all errors due to the inaccuracy of the scale 

 used were eliminated. The mean of a series of observations was taken as the 

 true length of the bubble. 



Plate XXI is a photograph of the bubble while in motion, and shows the 

 form it assumes while moving up the tube. 



Experiments were made with three classes of water, namely, water distilled 

 in vacuo, tap-water from the Dublin Vartry supply, and sea-water. In each 

 case, two experiments were made under conditions as similar to each other as 

 possible, though it was unavoidable that the atmospheric pressure and the 

 temperature varied a little. 



The manometer readings were in all cases converted to volumes at N. T. P., 

 and these included all the gases absorbed from the atmosphere during the 

 experiment. The effect of the C0 2 in the air is scarcely appreciable in the 

 case of the distilled water and of the sea- water. But in the case of the Vartry 

 water, the manometer observations indicate a greater absorption of gases than 

 in the case of the distilled water. The analyses of the gases extracted in the 

 two cases show that this increased absorption was due to the formation of 

 carbon dioxide in minute quantities in the Vartry water, and to the conse- 

 quent fixation of an equivalent volume of dissolved oxygen during the experi- 

 ment — a result, it may be assumed, due to slight oxidation of the humus in 

 the Vartry water. This water contains decided quantities of peaty matter. 1 



The actual experimental results obtained, in the case of distilled water, 

 tap-water, and sea-water, are recorded in Tables 1, 2, and 3. 



1 See the Experiments on the fermentative properties of humus in water, by one of 

 the authors. Trans. Roy. Dubl. Soc, 1895, pp. 593-615 ; and 1897, pp. 269-281. 



