540 Professor L. C. Miall [March 4, 



WEEKLY EVENING MEETING, 

 Friday, March 4, 1892. 



Basil Woodd Smith, Esq. F.E.A.S. F.S.A. Vice-President, in the 



Chair. 



Professor L. C. Miall, F.L.S. Professor of Biology at the 

 Yorkshire College, Leeds. 



Tlie Surface-film of Water, and its relation to the Life of Plants and 



Animals, 



It is necessary to the exposition of my subject that I should begin by 

 reminding you of some well-known properties of the surface of water. 

 These are familiar to every student of physics, and are set forth in 

 many elementary books. They are well explained and illustrated, 

 for instance, in Prof. Boys's deservedly popular book on " Soap- 

 bubbles." But there may be some persons here who have not quite 

 recently given their thoughts to this subject, and it will only cost us 

 a few minutes to repeat a few simple experiments, which will establish 

 some fundamental facts relating to the surface-film of water. 

 The following experiments were then shown : — 



(1) Mensbrugghe's float. Proves that the surface-film of water 

 offers resistance to the passage of a solid body from beneath. 



(2) Aluminium wire made to float on water. Proves that the 

 surface-film of water ofiers resistance to the passage of a solid body 

 from above. The resistance is proportional to the length of the line 

 of contact of the solid with the water. 



(3) Copper gauze made to float on water. Here, a number of 

 intersecting wires are employed instead of a single wire, and the 

 consequent increase in the length of the line of contact greatly 

 increases the weight which can be supported. 



(4) Frame with vertical threads, carrying a light plate of brass. 

 The threads hang vertically at first, but when the whole is dipped 

 into soapy water, the adhering film exerts a pull upon the sides of 

 the frame, draws the threads into regular curves, and raises the brass 

 plate. When the film is broken, the threads resume their previous 

 vertical position, and the j)late falls. 



(5) Aluminium wire supported by vertical copper wires. Each 

 end of the aluminium wire forms a loop, which fits loosely to one of 

 the copper wires. When the apparatus is dipped into soapy water, 

 the contraction of the film draws the aluminium wire upwards. 

 After pulling it down with a thread, the wire can be again drawn up. 

 This is another illustration of the tendency of the film to contract. 

 We use soapy water, because the fihn lasts for a coDsiderable time, 

 but the surface-film of pure water, though less viscous than that of 



