11)17] Soap Bubbles of Long Duration 179 



WEEKLY EVENING MEETING, 



Friday, January 19, 1917. 



His Grace the Duke of Northumberlan-d, K.G. D.C.L. F.R.S., 

 President, in the Chair. 



Professor Sir Ja^ies Dewar, LL.D. D.Sc. F.R.S., 



Fullerian Professor of Chemistry. 



Soap Bubbles of Long Duration. 



An inyestigation of the properties of Soap Bubbles is the logical 

 outcome of the experiments on flat liquid films described in the 

 1916 Friday Evening Discourse. Some fifty years ago Joseph 

 Plateau — Professor of Physics at Ghent — began a series of investi- 

 gations on Surface Tension Phenomena, and for nearly forty years 

 carried out a number of wonderful investigations, although prac- 

 tically blind, owing to an accident which occurred while examining 

 the action of intense light on the retina in the early part of his 

 career. It is due to tlie fascinating character of his pioneer work 

 on soap bubbles that the study has been continued. Since the 

 work of Plateau o-reat advances have been made in the experimental 

 study of Surface Tension Phenomena by Dupre, Mensbrugghe, 

 Reinold and Riicker, Michelson and his collaborators. The theo- 

 retical advancement which science owes to AVillard Gibbs dominates 

 all modern research on capillary phenomena. 



The flat films described last year were developed in an exhausted 

 atmosphere of nearly pure water vapour ; while soap bubbles are 

 blown up with air in air. Experiments extending over a year have 

 proved that in order to keep soap bubbles for any length of time 

 the air must be free from deteriorating gases and suspended solid 

 matters. The presence of living organisms and their spores amongst 

 the finely divided organic and inorganic particles in the atmosphere 

 was discovered by Pasteur. He purified the air by passing it 

 through a red-hot platinum tube containing platinum gauze, 

 afterwards removing the total carbonic acid. Later Tyndall 

 used a concentrated beam of light to demonstrate the presence of 

 floating impurities in the air, as a test of- "optical emptiness." 

 Tyndall passed a parallel beam from an arc lamp through a large 

 plano-convex lens, which condenses the light into a cone, at the apex 

 of which the intensity of the illumination causes the floating matter 

 in the air to be easily recognised. If a Bunsen or hydrogen flame is 

 placed below the focus, the reflected light gradually disappears as the 

 floating material, being largely organic, is burnt and passes into the 

 gaseous state. Another method used by Tyndall to obtain a dust- 



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