96 SMITH'S INTERMEDIATE CHEMISTRY 



ular units no longer exist. Thus, in sodium chloride, no sodium 

 atom can be said to be combined specifically with any one chlorine 

 atom. It is, instead, imprisoned by a number of chlorine atoms, 

 stationed at definite intervals around it, and among these chlorine 



FIG. 45 



atoms its combining forces must be regarded as impartially 

 dispersed. Very powerful forces, it will be evident, must be 

 called into play in order to constrain the separate atoms in a 

 crystal to retain their regular positions with respect to one another. 

 The nature of such forces will be discussed in a later chapter (see 

 p. 554-5). 



The resistance offered by the atoms in a crystal to forcible 

 changes of position is shown by the very low compressibility of 

 matter in the crystalline state. Nevertheless, some degree of 

 motion of the particles must still persist, since many crystalline 

 substances show a measurable vapor pressure. Some vapors, 

 indeed (e.g., phosphorus, iodine), can be condensed directly 

 to crystals without passing through the intermediate liquid state. 



The results of X-ray work indicate that this motion of the atoms 

 of a crystal consists of rapid vibrations about a mean position. 

 Atoms vibrating violently at the surface are evidently exposed to 

 the risk of breaking away altogether, after which they combine 

 to form molecules of vapor. Similarly, molecules of vapor strik- 

 ing the crystal surface may stick thereto, their constituent atoms 



