PHILOSOPHICAL SOCIETY OF WASHINGTON. 137 



the mass, adjacent molecules although entirely freed from the adhe- 

 sion which constitutes rigidity, yet (as has been shown by Joseph 

 Henry) preserve their mutual cohesion practically unimpaired : * 

 and hence devious as may be their wanderings, no portion of their 

 excursions can be called a free path. 



If the rapidity of the mean internal motion be still further accel- 

 erated until the momentum of the molecules is equal to their modu- 

 lus of " cohesion," the temperature of evaporation is reached, and 

 the molecules are impelled from their restraining bonds into a free 

 flight, which so long as undisturbed, continues (by the first law of 

 motion ) in an indefinite straight path in the direction of impulse. 

 The strength of these two bonds — adhesion and cohesion — differing 

 very widely in different substances, is thus measured by the amount 

 of kinetic energy absorbed in overcoming them, — the so-called 

 "latent heat" of fusion and of evaporation. In the case of ice, 

 the strength of the molecular adhesion is considerably less than 

 the sixth part of that of the cohesion. 



We thus perceive how the most solid bodies — even at low tem- 

 peratures — are exposed to surface evaporation without the oppor- 

 tunity of passing through the liquid state; since external molecules 

 from the great irregularity of their short oscillations, must occa- 

 sionally by the composition of motions from concurrent or imme- 

 diately successive shocks, acquire a velocity transcending the bonds 

 of cohesion, and thus escape entirely from the mass. 



We accordingly learn by the kinetic theory of gases that the dis- 

 crete or isolated molecules are flying about in all directions in straight 

 lines until by encounters with other molecules (or with material 

 barriers; their course is deflected. During the brief period of en- 

 counter (the disturbance of mutual encroachment), the trajectory 

 becomes a minute hyperbola. From the infinite variety of possible 

 impacts we also learn that each molecule must necessarily be con- 

 stantly changing within very wide limits the direction, the velocity, 

 and the length of its free excursions ; — even when a perfect equilib- 

 rium of temperature imports that the mean kinetic energy of the 

 entire system is constant and uniform. 



It is important for us to bear in mind that this wondrous theater 

 of continual intestine commotion does not present an example of a 



^Proceed. Am. Phil. Soc. April 5, & May 17, 1844: vol. iv, pp. 56, 57; 

 and 84, 85. 



