8d 



the threads being of such a length that the two matches form 

 the upper and lower lines of a square perpendicular plane. If 

 this be dipped into some soapy water it will come out bringing a 

 film of water with it, covering it like the skin of a drum. But 

 instead of the threads hanging vertically as they did before 

 immersion, they are curved inwards, and the bottom match is 

 raised up, showing that the surface film is exercising a pull and 

 endeavouring to contract. If the film be broken the lower bar 

 falls. The whole film consists of two skins with a layer of water 

 between them. The film is thus seen to behave like a piece of 

 stretched indiarubber, which is always trying to contract, 

 but it difters from stretched indiarubber in one respect. With 

 the stretched rubber the tendency to contract is difierent accord- 

 ing to how much it is stretched, while with a liquid film the 

 tendency to contract is always the same at the same temperature, 

 however much or little it may be stretched. Such films as the 

 above are plane surfaces, but liquid films may be also cylindrical 

 or spherical. A cylindrical film is easily obtained by placing 

 two wet wire rings in contact with each other, and lifting the 

 top one. If the bottom ring were not too heavy, the con- 

 tractile force of the cylindrical film would lift it up. The 

 power of this contractile force may be vividly manifested 

 in another manner. If two plates of glass six inches square 

 have a film of water between them in this way 1 -200th in. 

 thick, they are pulled together with a force about equal to 

 the weight of six pounds. Spherical films are seen in the 

 case of falling drops of liquid, partially spherical films in the 

 case of drops resting on a solid surface, where they are acted 

 on by their surface tension, tending to make them perfect 

 spheres, and the force of gravity tending to flatten them out. 



An interesting explanation was that which Dr. Draper gave 

 of the efi'ect of throwing a suitable kind of oil upon water to calm 

 it. Water being so much " thicker-skinned " than oil, the 

 latter, when cast upon it, immediately spreads out over the water 

 until the layer of oil gets so thin that it ceases to behave like 

 liquid oil at all. To this fact must be added that already alluded 

 to in the float experiment, that any other liquid mixed with water 

 lowers the resistance which the "skin "of the water ofters to 

 breaking. Suppose, then, a ship to be surrounded by a layer of 

 oil. The waves come rolling along towards it, reach the distant 

 edge of the oil film, there suddenly experience a lessening of the 

 tension of the surface, q,pd break there accordingly. The main 

 swell of the wave rolls on, but the ship is in the middle of a ring 

 of breakers, which surround it at the outer boundaries of the film 

 of oil. 



" On to a flat plate," said Dr. Draper, " we pour a thin 

 layer of coloured water, about 1-lOth or l-8th inch thick. On 



