MOLECULAR CONSTITUTION OF SOLIDS. 151 



path will become still more unobstructed if the ends which are 

 ed together are somewhat sharpened off at the edges. 

 At a, where the two parts of the groove adjoin, a piece of cork cut 

 so as to fit into the triangular space between them and the board is 

 pushed underneath and glued to the cardboard ; the whole may 

 then be fixed to the board by a stout pin stuck through the cork ; 

 the upper end of the groove may lean loosely on the upright or be 

 fastened lightly by a pin or a tack. 



17. Molecular Constitution of Solids. Rigidity. Elasti- 

 city. It has been stated already, in art. 4, that the 

 cause of the resistance which solid bodies offer to the 

 separation of their particles is termed cohesion. Cohe- 

 sion is an attractive force, acting between the ultimate 

 molecules of a solid body; but it differs from other 

 attractive forces for example, the force of gravitation 

 in this respect, that while gravity acts between bodies 

 at all distances, cohesion acts only when the distance 

 between the particles is exceedingly small. The pieces 

 into which a body may be broken up by any external 

 force will not again cohere if placed in contact. The 

 molecules have undergone a certain amount of mutual 

 displacement by the action of the external force, and 

 cannot again be brought so close together as they were 

 previously; they cannot again be brought under the 

 influence of cohesion, which acts between molecule and 

 I molecule at an indefinitely small distance only: the 

 body remains therefore divided. There are other forces, 

 s besides cohesion, which act only between molecules at 

 indefinitely small distances, and are hence termed mole- 

 cular forces ; such are elasticity and adhesion. 



The resistance opposed by the molecules of a solid 

 body to any external force which tends to overcome 

 their cohesion, is also called the rigidity of a body. The 

 rigidity depends on the manner in which the external 



