THE SYPHON. 65 



have said, will be higher in the tube, and concave in form ; but mercury will 

 be depressed below the outside level, and convex, because mercury will not 

 adhere to glass. When- the force of cohesion to the sides of the tube is more 

 than twice as great as the adhesion of the particles of the liquid, it will rise 

 up the sides, and if the forces be reversed, the rounded appearance will follow. 

 This accounts for the convex appearance, or " meniscus," in the column of 

 mercury in a barometer. 



Amongst the complicated experiments to demonstrate molecular attrac- 

 tion, the following is very simple and very pretty : Take two small balls of 

 cork, and having placed them in a basin half-filled with water, let them come 

 close to each other. When the}' have approached within a certain distance 



Fig. 67. An improvised syphon. 



they will rush together. If you fix one of them on the blade of your pcn- 

 knife> it will attract the other as a magnet, so that you can lead it round 

 the basin (fig. 68). But if the balls of cork are covered with grease they will 

 repel each other, \vhich fact is accounted for by the form of the menisqucs, 

 which are convex or concave, according as they are moistened, or preserved 

 from action of the water by the grease. 



This attribute is of great use in the animal and vegetable kingdoms. 

 The rising of the sap is one instance of the latter. 



Experience in hydrostatics can be easily applied to amusing little 

 experiments. For instance, as regards the syphon, we may make an image 



5 



