7s6 Professor Sir James Dewar [Jau 



AVEEKLY EVENING MEETING, 



Friday, January 21, 1916. 



.His Grace the Duke of Northumberland, P.O., K.G., LL.D., 

 ' F.R.S., President, in the Chair. 



Professor Sir James Dewar, M. A., LL.D., D.Sc, F.R.S., M.R.I., 

 Fullerian Professor of Chemistry. 



Problems in Capillarity. 



Studies in gas diffusion described in last year's discourse have lead 

 to the examination of films of great tenuity for such enquiries. 

 These films can be got in the solid state through the use of solu- 

 tions of nitro-cotton in certain solvents, or in the liquid state by 

 employing Plateau's soap solution. The object of the present 

 Discourse is to describe the result of some attempts to produce 

 apparatus suitable for the study of the properties of such films, along 

 with an account of the results so far obtained. 



Liquid films thin enough to show colours are produced when 

 drops of a large number of organic substances are allowed to touch 

 the surface of pure water. The apparatus adapted for demonstrating 

 the colour bands and also for making solid films is shown in Fig. 1. 

 A copper funnel A of 10 cm. diameter, blackened by coating with 

 nitrate of copper and subsequent heating to redness, is connected to the 

 Institution water supply in order to ol)tain an easily renewable and well- 

 defined clean water surface. The funnel is fixed in a blackened hollow 

 metal conical stand B shown in section ; this stands on a shelf over a 

 sink. A parallel beam of light is directed down at about 40° inclina- 

 tion, and, after reflection from the liquid surface in the funnel, is 

 focused on the screen. The best definition is obtained by allowing 

 the water surface to fall a little ]>elow the rim of the funnel, for 

 which purpose a two-way tap is connected below^ It is essential 

 • that a fresh surface be produced by flooding with water between 

 each experiment. A small drop of a suitable liquid on the end of a 

 glass rod is made to touch the water in the funnel near the centre 

 of the surface. The difference of surface tension between the water 

 and the liquid drop results in the latter being spread to a circular 

 film tliin enough to produce colour bands in the same definite 

 order exhibited by soap films. The production and develop- 

 ment of the colouring is generally very rapid, so that the final 

 thinnest stage of blackness is quickly reached. Pinene, turpentine, 



