6 4 



SCIENTIFIC RECREATIONS. 



phenomena, etc., do not need any special appliance for demonstration, and it 

 is the same with the convexity or concavity of meniscuses. 



Fig. 65 represents a pretty experiment in connection with these 

 phenomena. I take a glass, which I fill up to the brim, taking care that 

 the meniscus be concave, and near it I place a pile of pennies. I then ask 

 my young friends how many pennies can be thrown into the glass without 

 the water overflowing. Everyone who is not familiar with the experiment 

 will answer that it will only be possible to put in one or two, whereas it- is 

 possible to put in a considerable number, even ten or twelve. As the 

 pennies are carefully and slowly dropped in, the surface of the liquid will 



be seen to become more and more convex, and one 

 is surprised to what an extent this convexity 

 increases before the water overflows. 



The common syphon may be mentioned here. It 

 consists of a bent tube with limbs of unequal length. 

 We give an illustration of the syphon (fig. 66). The 

 shorter leg being put into the mixture, the air is ex- 

 hausted from the tube at <?, the aperture at g being 

 closed with the finger. When the finger is removed 

 the liquid will run out. If the water were equally 

 high in both legs the pressure of the atmosphere would 

 hold the fluid in equilibrium, but one leg being 

 longer, the column of water in it preponderates, and as it falls, the pressure 

 on the water in the vessel keeps up the supply. 



Apropos of the syphon, we may mention a very simple application of 

 the principle. Cut off a strip of cloth, and arrange it so that one end shall 

 remain in a glass of water while the other hangs down, as in the illustration. 

 In a short time the water from the upper glass will have passed through the 

 cloth-fibres to the lower one (fig. 67). 



This attribute of porous substances is called capillarity ', and shows itself 

 by capillary attraction in very fine pores or tubes. The same phenomenon is 

 exhibited in blotting paper, sugar, wood, sand, and lamp-wicks, all of which 

 give familiar instances of capillarity. The cook makes use of this property 

 by using thin paper to absorb grease from the surface of soups. 



Capillarity (referred to on page 25) is the term used to define 

 capillary force, and is derived from the word capillus, a hair; and so very 

 small bore tubes are called capillary tubes. We know that when we 

 plunge a glass tube into water the liquid will rise up in it, and the narrower the 

 tube the higher the water will go ; moreover, the water inside will be higher 

 than at the outside. This is in accordance with a well-known law of adhesion, 

 which induces concave or convex surfaces in the liquids in the tubes, accord- 

 ing as the tube is wetted with the liquid or not. For instance, water, as we 



Fig. 66. The Syphon. 



* The curved surface of a column of liquid is termed a " meniscus," from the Greek word 

 i meaning " a little lens." 



