260 Mr. G. T. Beilby on the Hard 
passed permanent deformation is produced, some of the 
molecules being moved into new positions relatively to each 
other, positions which are not in strict accord with the 
geometrical molecular arrangement. 
8. But in cozsidering the transition from the liquid to the: 
solid state we are not necessarily limited to the crystalline 
form of aggregation. Ina former paper I have shown that 
the forms assumed by the surface layer of a solid metal during 
annealing are controlled by surface tension rather than by 
erystallic force*. In this case the ordinary forces of cohesion 
determine the positions taken up by the molecules relatively 
to each other. Throughout a layer, which is probably many 
molecules in thickness, the molecules have come to rest in the 
midst of the activity of the liquid state, their mutual relations. 
in that state being stereotyped at the moment of cooling. 
9. The passage from the liquid to the solid state consists 
essentially in the arrest or the limitation of the freedom of 
movement of the molecules relatively to each other. This 
freedom of movement is determined by the temperature of 
the substance. When the temperature is reduced by the 
withdrawal of heat, the molecules must come to rest in the 
solid state. If the arrest is instantaneous, then the molecules. 
will have no opportunity to arrange themselves according to: 
their polarity and the solid state which results will be a 
heterogeneous assemblage of molecules which will bear some-. 
what the same relation to the homogeneous assemblage of a 
erystal as a crowd of disbanded soldiers does to the same 
men in the orderly formation of the company or the battalion. 
10. A solid of this type may be likened to an instantaneous 
photograph of the molecules in the liquid state ; but while 
in the photograph there is only the appearance of fixity, in. 
the congealed liquid the fixity is real. 
11. In addition to the inherent polarity of the molecules, 
erystalline formation requires time, freedom, and space. Time. 
and freedom are necessary to enable the molecules to turn 
into the proper orientation; and space may be required if the- 
packing of the molecules in the homogeneous assemblage is 
more open than it is in the heterogeneous. 
12. The rigidity of the solid state is therefore due to the 
arrest of the molecular movement either in a homogeneous 
or in a heterogeneous assemblage of molecules. Throughout 
this paper the term crystalline is applied to the former and 
amorphous to the latter. 
13. We can picture the transition from the liquid to the 
amorphous solid state as taking place either by a sudden 
*" Proce. Roy. Soc. vol. xxii. p. 227. 
