72 ELEMENTS OF SCIENCE 



(as in the case of water in a glass vessel) then the surface 

 of the liquid will rise at its circumference so that its 

 upper surface is concave. If the liquid be much denser 

 than the vessel containing it (as with mercury contained 

 in glass), then the surface of the liquid will be slightly 

 depressed at its circumference and the upper surface of 

 the mercury will therefore be convex. 



This attraction, which is due to the surface contact of 

 the liquid and solid, is shown on plunging any solid body 

 into water, when some of the water adheres to it and comes 

 out wet. If thin plates of different substances be made to 

 touch the surface of water, considerable force is required 

 to raise them from it, and the amount of force thus 

 required varies in amount with the nature of the sub- 

 stance employed. One conspicuous form of such action 

 is that which is known as capillary attraction. This is 

 the attraction exercised by tubes of very fine bore upon 

 liquids into which their ends may be plunged. 



If any substance containing minute canals of the kind, 

 be immersed in water, then the water will ascend them 

 to a height which will be the greater the narrower the 

 cavities it ascends. As examples of such action may be 

 taken the small cavities in blotting paper, sponge, the 

 cotton of a lamp, lump sugar, &c., in all of which liquid 

 will readily ascend. It is because such cavities are 

 generally not broader than a human hair that this 

 attraction has been termed " capillary." 



The motions of liquids constitute another section of 

 mechanics known as hydrodynamics or hydraulics. 

 Since liquids can be set in motion with so much greater 

 facility than solids, and since the direction and velocity 

 of their movements are liable to modification by so 

 many causes which would not modify the action of mov- 

 ing solids, it is evident that the conditions of their 



