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VII. The Kinetic Properties of a Molecule in a Substance. 

 By K. D. Kleeman, ~D.Sc, B.A., Clerk- Maxwell Student 

 of the University of Cambridge *. 



The Minimum Velocity of a Molecule. 



IT is usually stated as an assumption that the kinetic 

 energy of a molecule in a liquid is the same as that 

 in a gas at the same temperature. It is possible to obtain 

 definite information on this point, from which it will appear 

 that this loose statement must be considerably modified. 

 This point has already been investigated in a previous 

 paper f . Since the result obtained forms the foundation 

 of the formulae deduced in this paper, the investigation will 

 in part be repeated. 



Suppose that a thermometer is placed in a perfect gas. 

 It will assume the temperature of the gas, through being 

 bombarded by the gas molecules. From experience we 

 know that if the gas is allowed to expand without doing 

 external work, the temperature of the thermometer is not 

 altered, showing that the temperature it indicates is inde- 

 pendent of tlie number of molecules impinging per second 

 per cm. 2 of *its surface. Let us consider the effect of the 

 attraction of the material of the bulb on the molecules of 

 the gas. AVhen a molecule gets into the zone of attraction 

 of the bulb its velocity is increased, and each molecule thus 

 impinges on the bulb with a greater velocity than it possesses 

 when not under the influence of an external force. Kow 

 suppose the bulb covered with a material which exerts a 

 much more powerful attraction on the surrounding gas 

 molecules than the material of the bulb. Each molecule 

 now impinges on the bulb with a much greater velocity 

 than before. But we know from experience that the 

 temperature indicated by a thermometer is independent 

 of the material of the bulb. It appears, therefore, that the 

 temperature indicated by a thermometer placed in a gas 

 is regulated by the velocity of the molecules when not under 

 the influence of an external force. 



Suppose the thermometer is placed in a liquid. The 

 temperature of the liquid and thermometer are equalized, 

 as before, by the molecules of the liquid bombarding the 



* Communicated by the Author. 



t Proc. Camb. Phil. Soc. vol. xvi. pt. vi. pp. 540-559. This paper 

 deals with the properties of the different internal energies into which 

 the internal energy of a substance can be divided, and the nature of the 

 undetermined function in the law of molecular attraction. 



