COHESION AND ADHESION. 



each other at very small distances apart. Cohesion has received various 

 names in order to express its various degrees. For instance, we say a body 

 is tough or brittle, or soft or hard, according to the degrees of cohesion the 

 particles exercise. We know if we break a glass we destroy the cohesion; 

 the particles cannot be reunited. Most Liquid particles can be united, but 

 not all. Oil will not mix with water. 



The force ,of cohesion depends upon heat. Heat expands everything, 

 and the cohesion diminishes as temperature increases. 



There are some objects or substances upon the earth the particles 

 of which adhere much more closely than others, and can only, with very 

 great difficulty, be separated. These are termed Solids. There are other 

 substances whose particles can easily be divided, or their position altered. 

 These are called Fluids. A third class seem to have little or no cohesion 

 at all. These are termed Gases. 



Adhesion is also a form of attraction, and is cohesion existing on the 

 surfaces of two bodies. When a fluid adheres to a solid we say the solid is 

 wet. We turn this natural adhesion to our own purposes in many ways, we 

 whitewash our walls, and paint our houses ; we paste our papers together, etc. 



On the other hand, many fluids will not adhere. Oil and water have 

 already been instanced. Mercury will not stick to a glass tube, nor will 

 the oiled glass tube retain any water. We can show the attraction and 

 repulsion in the following manner : Let one glass tube be dipped into water 

 and another into mercury, you will see that the water will ascend slightly 

 at the side, owing to the attraction of the glass, while the mercury will 

 be higher in the centre, for it possesses no attraction for the glass (fig. 17). 

 If small, or what are termed capillary (or hair) tubes, be used (fig. 18), the 

 water will rise up in the one tube, while in the other the mercury will 

 remain lower than the mercury outside the tube. (See Capillarity) 



Fig. i8 



Chemical Attraction is the force by which two different bodies unite to 

 form a new and different body from either. This force will be fully con- 

 sidered in CHEMISTRY, m a future part. 



It is needless for us to dwell upon the uses of these Forces of Nature. 

 Gravity and Cohesion being left out of our world, we can imagine the result. 

 The earth and suk and planets would wander aimlessly about ; we should 

 float away into space, and everything would fall to pieces, while our bodies 

 would dissolve into their component parts. 



T/tc Balance and Centre of Gravity. We have spoken at some length 

 about Gravity, and now we must say something respecting that point called 



