82 



The next thing he showed was a bowl containing wires, 

 darning needles, and snch-like articles, composed of metal seven 

 or eight times as heavy as water, all floating about as happy as 

 could be. Nevertheless, it is easier in some respects to launch a 

 torpedo-destroyer than a darning needle. The best method is to 

 grease it slightly, take it tenderly on the prongs of a fork^he 

 needle, not the torpedo-destroyer — and lower it gently, keeping it 

 perfectly horizontal. 



After a reference to the familiar spectacle of water-flies 

 skimming the surface of a pool. Dr. Draper came to another 

 illustration of his subject. If a sheet of muslin be carefully 

 stretched over a ring held horizontally and water be gently placed 

 upon it, by means of a pipette or syringe, a large flattened drop 

 may be obtained on the muslin without any of the water passing 

 through. The surface film touches the muslin, which is not 

 really wetted by the water. And so long as the skin remains 

 unbroken the water does not pass through, and the muslin 

 remains dry. If once the skin be broken the threads of muslin 

 become wet and the drop or part of it passes to the under surface 

 of the muslin, and if the drop there formed be too heavy it 

 falls oft", and so a stream is started which will carry nearly all the 

 water away. In this case the water does not, however, all escape ; 

 there is a skin above the muslin and another forms below, and a 

 quantity of water is held imprisoned between them. People who 

 live in tents in rainy weather often have decided experience of 

 these matters. When a steady rain falls on canvas it does not 

 at first make its way through. A film spreads all over the 

 material, and so long as this skin remains unbroken all the 

 interstices between the material are filled up and no wet gets 

 through. But the slightest touch at any spot brings the film 

 through to the inside, and then the water will run through as 

 though a hole were cut in the canvas. The use of umbrellas is 

 based on the same principle, and their effectiveness is limited in 

 the same way. 



Though all liquids have films of this kind, yet all the films 

 are not of the same strength. Ether, for instance, and alcohol, 

 are much more " thin-skinned." If Mensbrugghe's float be 

 submerged as already described in water, and ether or alcohol be 

 allowed to become mixed with the water, the float immediately 

 breaks its way through. The surface tension of water is greater 

 than that of any other liquid except mercury. 



Dr. Draper then described a number of experiments devised 

 to show other properties possessed by the *' skin " besides 

 that of resistance to penetration. The most characteristic of 

 these is its contractility, which may be observed in many 

 different ways. A good method is to suspend one lucifer match 

 horizontally from another by means of a thread at each end, 



