Molecular Attraction 47 



with such force that all stay in their places. Substances 

 in which they pull the hardest, like steel, are very hard 

 to break in two ; that is, it is difficult to pull the mole- 

 cules of these substances apart. In liquids, such as 

 water, the molecules do not pull nearly so hard on each 

 other. In a gas, such as air, they are so far apart 

 that they have practically no pull on each other at all. 

 That is why everything would turn to a gas if the force 

 of cohesion stopped. Why things would turn cold will 

 be explained in Chapter 4. 



Cohesion, adhesion, and capillary attraction, all are 

 the result of the pull of molecules on each other. The 

 difference is that capillary attraction is the pulling of 

 particles of liquids up into fine spaces, as when a lamp 

 wick draws up oil ; adhesion is the pull of the particles 

 of one substance or thing on the particles of another 

 when they are very close together, as when water clings 

 to your hand or when dust sticks to the ceiling ; while 

 cohesion is the clinging together of the particles of the 

 same substance, like the holding together of the particles 

 of your chair or of this paper. 



When you put your hand into water it gets wet be- 

 cause the adhesion of the water to your hand is stronger 

 than the cohesion of the water itself. The particles 

 of the water are drawn to your hand more powerfully 

 than they are drawn to each other. But in the following 

 experiment, you have an example of cases where cohesion 

 is stronger than adhesion : 



Experiment 16. Pour some mercury (quicksilver) into a 

 small dish and dip your finger into it. As you raise your 

 finger, see if the mercury follows it up as the water did 



