18 THE PROPERTIES OF MATTER. 



cules ; adhesion is the force that holds together 

 unlike molecules. 



(a.) Cohesion is the force that holds most substances 

 together and gives them form. Were cohesion suddenly 

 to cease, brick and stone and iron would crumble to finest 

 _., powder, and all our homes and cities and selves fall to 



hopeless ruin. In aeriform bodies, cohesion is not ap- 

 parent, being overcome by molecular repulsion (heat). In 

 large masses of liquids the cohesive force is overcome by gravity, 

 which tends to bring all the molecules as low as possible and thus 

 renders their surfaces level. But in small masses of liquids, the 

 cohesive force predominates and draws all the molecules as near 

 each other as possible, and thus gives to each mass the spheroidal 

 form, as in the case of the dew or rain-drop. Globules of mercury 

 upon the hand or table, and drops of water upon a heated stove, are 

 familiar illustrations of this effect of cohesion upon small liquid 

 masses. But in the solid state of matter, cohesion shows most 

 clearly. Cohesion acts only at ijr sensible (molecular) distances. Let 

 the parts of a body be separated by a sensible distance, and cohesion 

 ceases to act ; we say that the body is broken. If the molecules of 

 the parts can again be brought within molecular distance of each 

 other, cohesion will again act and hold them there. This may be 

 done by simple pressure, as in the case of wax or freshly-cut lead ; 

 it may be done by welding or melting, as in the case of iron. Cir- 

 cular plates of glass or metal, about three inches in diameter, often 

 have their faces so accurately fitted to each other that, when pressed 

 together, a considerable force is needed to separate them. (See 

 Fig. 4.) 



(&.) Adhesion is the force that causes the pencil or crayon to leave 

 traces upon the paper or blackboard, and gives efficacy to paste, 

 glue, mortar and cements generally. In a brick wall, cohesion binds 

 together the molecules of the mortar layer into a single, hardening 

 mass, while on either hand adhesion reaches out and grasps the ad- 

 joining bricks and holds them fast a solid wall. Like cohesion, it 

 acts only through distances too small to be measured ; unlike cohe- 

 sion, it acts between unlike molecules. 



47. What is Hardness ? Hardness is that 

 property of matter by virtue of which some bodies 

 resist any attempt to force a passage between their 

 particles. 



