14 ELEMENTARY GENERAL SCIENCE CHAP. 



The application of a comparatively small force to the small one 

 is felt on the larger one, and it is as many times greater in an 

 upward direction as the piston of B is larger in area than the 

 piston in A. This great upward force is being used in the 

 instance shown in the diagram to compress bales of wool. 



Liquids can be separated into Drops which will run 

 together again. Liquids are able to form drops because of a 

 property, known as cohesion, which they possess. The larger 

 the drop which can be formed the greater is the cohesion between 

 the particles of the liquid. 



Cohesion is the mutual attraction which the molecules of a body 

 exert upon one another ; it is, indeed, the force which keeps the 

 particles of a substance together. Cohesion is strongest in solids, 

 which in its absence would crumble into powder ; it acts 

 between the particles of liquids, but in gases it may said to be 

 absent. 



The following experiments beautifully show this property of 

 liquids : 



EXPT. 16. Sprinkle some powdered resin on a board arid 

 then a little water. Notice the water collects in drops ; the 

 smaller they are, the more nearly spherical they are. Observe 

 the same thing with mercury on a sheet of paper. 



EXPT. 17. Mix methylated spirit and water until a few 

 drops of oil just float when the mixture is quite cool. Pour 

 fresh oil, by means of a pipette, into the middle of the mixture. 

 Notice spherical globes of oil can thus be formed. 



EXPT. 18. Observe that drops once formed can be made to 

 run together again by coming in contact. 



It will be as well before leaving this subject to learn in what 

 way cohesion differs from another similar property possessed by 

 some kinds of matter and which is known as adhesion. Adhesion 

 denotes the attraction between unlike particles of matter. A 

 metal plate may be made to adhere to glass ; and a postage stamp 

 may be said to exhibit the existence of the force of adhesion when 

 it is stuck upon an envelope. 



Another experiment will make clear the difference between 

 this adhesion exhibited between the postage stamp and the 

 envelope and the force of cohesion : 



EXPT. 19. Carefully clean and polish two pieces of plate 



