94 SMITH'S INTERMEDIATE CHEMISTRY 



the available space, but forjns coherent drops, the curvature 

 of the surface of which indicates that tremendous forces are 

 existent, pulling the outside molecules towards the interior of 

 the liquid. Nevertheless, molecules do escape outwards from a 

 liquid surface to form vapor (see p. 62^). We must therefore 

 conclude that the molecules of a liquid are still in rapid motion. 

 Similarly, when liquids which are capable of mixing (e.g., alcohol 

 and water) are placed in separate layers in the same vessel, they 

 do mix, slowly, by diffusion. The rate of dispersion of the mole- 

 cules, although much impeded by their close packing, has not 

 been annihilated. 



A liquid still possesses, therefore, in a modified degree, many 

 of the properties exhibited by a gas. For any particular substance, 

 the differences between the behavior of the liquid and the gaseous 

 states grow less and less as the temperature is raised, until at 

 the critical temperature (see p. 91) the two states become identical. 



The Properties of Solids. True solids are sharply distin- 

 guished from liquids by their crystalline forms (Figs. 39-43, see 



FIG. 39 FIG. 40 FIG. 41 FIG. 42 FIG. 43 



Octahedron Square Pris- Rhombic Monosymmetric Asymmetric 

 (Alum) matic (Niter) (Gypsum) (Hydrated cupric 



sulphate) 



also pp. 4, 5 and 12), which possess definite planes of cleavage and 

 by their behavior towards light and X-rays give further evidence 

 of regular structure. Substances such as glass, which do not exhi- 

 bit any specific crystalline form, although commonly called solids, 

 are strictly speaking still in an extremely viscous liquid state. 

 In such substances, as in liquids and gases, the cohesive forces 



