1895.] Note on the Motions of and within Molecules* fyc. 177 



IV. "Note on the Motions* of and within Molecules; and on 

 the Significance of the Ratio of the two Specific Heats 

 in Gases." By G. JOHNSTONS STONEY, M.A., D.Sc., F.R.S. 

 Received April 29, 1895. 



Take first a manifestly phosphorescent substance, such as calcium 

 sulphide. For a long time after exposure to sunshine such a body 

 retains the power of glowing in the dark, and the spectrum of the 

 light which it then emits consists of definite lines or bands. 



The temperature of the phosphorescent substance may all this time 

 be low when tested by the thermometer, i.e., by its power of commu- 

 nicating heat to bodies with which it is in contact. 



Let us now apply another test of temperature. Imagine the phos- 

 phorescent body to be placed close outside one face of a closed 

 chamber. Let the other sides of the chamber be adiabatic, while 

 that next the phosphorescent substance is a window glazed with a 

 material transparent to the phosphorescent rays, and opaque to all 

 other radiations; and let there be a theoretically perfectly black 

 solid within the chamber. 



Such a solid emits all the rays due to its temperature, and them 

 only none others, however much a luminary may shine upon it. 



Let us suppose that two specimens of the phosphorescent substance 

 are provided, which can be alternately exposed to sunshine, and 

 placed so as to radiate into the closed chamber through its window. 

 In this way the phosphorescent radiation on to the black body within 

 can be kept up for any desired length of time. 



Let the black body at the commencement and the phosphorescent 

 substance before its exposure to sunshine be at the ordinary atmo- 

 spheric temperature. Now let the phosphorescent body be excited 

 by sunshine, and put into its position in front of the Avindow of the 

 closed chamber. The light emitted by the phosphorescent substance 

 will then radiate into the chamber without equivalent exchange. 

 Accordingly the temperature of the black body within will rise, and 

 will continue rising until its temperature has reached something like 

 white heat: in fact until that point is reached at which a glowing 

 black body radiates luminous rays which, at the particular parts of 



* The term motions as used in this paper is to be understood in a sense, both 

 generalised and limited, as including along with the motionc proper of ponderable 

 matter, any other event of or within the molecule in which energy arriving from 

 outside can be stored whether in the form of potential energy of ponderable 

 matter, or in motions or configurations of the electrons, or in any other way. 

 It does not include any event which is completely isolated, if such exist within the 

 molecule, i.e., any event which is wholly incapable of exchanging energy with the 

 other events going on within and outside the molecule. 



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