14 NATURAL HISTORY OF AQUATIC INSECTS 



the prongs of a fork, and lower the fork and needle 

 steadily upon the surface of the water in a basin. If 

 the fork is taken away from beneath, the needle will 

 be left floating on the surface. Steel is many times 

 heavier than water, but the resistance of the surface- 

 film prevents the needle from sinking. Once beneath 

 the surface, the needle sinks rapidly to the bottom. 



The surface-film is in a state of tension, i.e. it exerts 

 a pull. It behaves like a drum-head, or a sheet of 

 stretched india rubber. This may be made evident 

 in various ways. One of the simplest is this. Take 

 two lucifer matches, which are to form opposite sides 

 of a square. Tie their ends together with pieces of 

 sewing-cotton, which will form the remaining sides. 

 Then run a needle into the middle of one of the 

 matches to make a handle. If this frame is dipped 

 into soapy water (a strong solution is required) and 

 then lifted out, a transparent film of soapy water will 

 overspread the square. This film will be in a state of 

 tension, as you will see by holding the frame upright 

 with the two matches horizontal. The threads will 

 be curved inwards by the pull of the film. Why do 

 we use soapy water in this experiment ? Because the 

 film of pure water will not last long enough for us to 

 examine it. It breaks up instantly into a crowd of 

 small drops. The film of soapy water, lifted off in 

 this way, is not a simple surface-film. It consists of 

 a double film, with an appreciable quantity of water 

 imprisoned between the two layers. 



The two or three experiments just described will, 

 if actually performed, establish in the reader's mind 

 some elementary truths, which have a good deal to do 



