Prof. Norton on Molecular Physics. 383 



nished force from the sides at which the atmosphere is expanded. 

 This follows, as a necessary consequence, from the fundamental 

 conception of molecular forces, developed on pages 199 to 

 204, as may be distinctly seen by attending to the values of 



71 



— and r. It will thus be seen that the molecular atmospheres, 



in assuming the spheroidal form, under special circumstances, 

 determine the existence of molecular axes of cohesive attraction, 

 u whose force is inversely related to their length." In the direc- 

 tion of such axes, then, the limit of stable equilibrium (Oa, 

 fig. 1, p. 203) will be least for the shorter axis, and greater for 

 the longer axis. 



When a force of pressure applied to a body determines a per- 

 manent compression, the molecular atmospheres remain com- 

 pressed, or in closer proximity to their central atoms, and the 

 force of cohesive attraction is permanently increased, and the 

 limit of stable equilibrium diminished. The heat developed is 

 a necessary result of the compression of the atmospheres. If the 

 elastic reaction to the pressure after it is withdrawn were per- 

 fect, the atmospheres would resume their original form and 

 dimensions, the origiual molecular forces would be recovered, 

 and the heat evolved would be absorbed again. In general 

 when mechanical forces are applied to a body, the heat evolved, 

 or absorbed, is a necessary accompaniment of the compressions, 

 or expansions, superinduced in the atmospheres of the particles, 

 and may be regarded as a sensible indication of the extent of 

 such changes of molecular condition. The mechanical work, of 

 which the heat evolved serves as the measure, is expended in 

 urging the atmospheres nearer to their central atoms. 



On the other hand, if heat be directly applied to a body, it 

 has a tendency opposite to that of a mechanical pressure, or to 

 expand the molecular atmospheres, and so to reduce the inten- 

 sity of the cohesive attraction at a given distance. It is to be 

 observed also that heat has a tendency to dissolve the groups in 

 which the particles of a solid may be aggregated, and, when the 

 point of fusion is reached, will effectually break up such groups 

 and bring the mass to the condition of a homogeneous and sym- 

 metrical arrangement of molecules. 



Solidification, or Crystallization. — It is a well recognized prin- 

 ciple that solidification and crystallization are the same process. 

 This great principle was first propounded by the learned and 

 acute Dr. Young in 1807, in his lectures on Natural Philosophy. 

 It has also been advocated by Biot and other physicists, and 

 more recently has been reasserted and ably sustained by Professor 

 Dana, in his admirable paper " On Cohesive Attraction," pub- 

 lished in vol. iv. second series, of Silliman's Journal. If now it 



