1917] 



on Soap Bubbles of Long Duration 



203 



the normal thinning of the biibl)le to the black stage is prevented. 

 There is instead an increase in thickness, causing gradational changes 

 of colour : the bubble sags down from the increase in weight ; suc- 

 cessive drops accumulate and fall off, at first, maybe, two or three in 

 a day, decreasing rapidly in number as the dilution proceeds, until 

 when the soap-glycerine is reduced to 1 or 2 per cent, the interval 

 between the drops grows into many weeks, and the drainage practically 



Fig. 11. 



ceases. When very diluted the black stage is finally reached, and may 

 remain, provided the temperature is steady ; but usually much fluctua- 

 tion of colour takes place, the black even recurring many times. 



The washings of several such bubbles were collected in a graduated 

 tube, and periodically weighed and analysed. In the case of a bubble 

 of glycerine and ammonium oleate, the analysis for most purposes 

 consisted simply in heating the liquid— in an oven at 90°-95' C.-- 

 until constant in weight. " The contained glycerine and oleic acid 



