136 BULLETIN OF THE 



setherial undulation, the differences of whose manifestations de- 

 pend entirely upon the uature of the body, organic or inorganic, 

 on which it falls.* 



Molecular Thermo-dynamics. — Passing from the wave theory of 

 radiation to the related subject of the internal re-actions of bodies, 

 the application of thermo-kinetics to the facts of temperature has 

 taught us that the molecules of all bodies are in a state of very 

 rapid though minute movement, and that this movement, while 

 being constantly transferred and expended, (and thus ever tending 

 to the absolute zero,) is yet incessantly maintained in varying quan- 

 tity by repeated re-enforcements from natural and artificial sources 

 of heat, and by mutual interchanges. In the case of solid bodies, 

 whose constituent molecules are held together by what we must call 

 (in default of any names as yet invented by the kinematist) the 

 qualities of cohesion and adhesion, — their mutual contact being re- 

 sisted and prevented by what we must for the present call a repel- 

 lant quality, the temperature motion is in the nature of an oscilla- 

 tion or rather irregular reverberation within the narrow limits of 

 opposite resistances, by which the relative mean position of the 

 particles and the stability of the body are preserved. By the term 

 "cohesion" is designated simply the observed fact of a resistance 

 to divellent or tensile stress; by the term "adhesion" is designated 

 the observed fact of resistance to torsional or shearing stress. 



When the energy of the molecular movements is increased until 

 the modulus of "adhesion" is equalled, the point of melting is 

 reached, and the molecules instead of being restored to their ante- 

 cedent positions are carried irregularly from the influence of neigh- 

 bor to neighbor, and thus become fluent by being deflected among 

 each other in all possible directions. In this " liquid " condition of 



*Am. Jour. Sci. Jan. and Feb., 1873: vol. V, pp. 25-38, and 91-98. 

 Dr. Draper's results (so far as the refrangibility of radiant heat is con- 

 cerned) have recently been confirmed by the refined investigations of Prof. 

 S. P. Langley, by means of his "actinic balance." (Proceed. Am. 

 Acad. Jan., 1881: vol. xvi, p. 342; Am. Jour. Sci. March, 1881: vol. 

 XXI, p. 187; Nature. Oct. 12, 1882: vol. xxvi, p. 588.) 



"A ray of specified wave-length and specified plane of polarization, can- 

 not be a combination of several different things, such as a light-ray, a heat- 

 ray, and an actinic ray. It must be one and the same thing, which has 

 luminous, thermal, and actinic effects." J. Clerk Maxwell. (Theory 

 of Heat. 1872: chap, xvi, p. 218.) 



