TAITS "THERMODYNAMICS." 665 



from what are called the First and Second laws of Thermodynamics, without 

 an J hypotheses as to the molecular constitution of bodies, all speculations as 

 to how much of the energy in a body is in the form of heat are quite out 

 of place. 



Prof. Tait, however, does not seem to have noticed that Prof. Clausius, in 

 a footnote to his sixth memoir *, tells us what he means by the heat in a 

 body. In the middle of a sentence we read : 



" the heat actually present in a unit weight of the substance in question in other words, 



the vis viva of its molecular motions" 



Thvis the doctrine that heat consists of the vis viva of molecular motions, 

 and that it does not include the potential energy of molecular configuration 

 the most important doctrine, if true, in molecular science is introduced in a 

 footnote under cover of the unpretending German abbreviation "d.h." 



Prof. Clausius is himself the principal founder of the kinetic theory of 

 gases. The theory of the exchanges of the energy of collections of molecules 

 was afterwards developed by Boltzmann to a much greater extent than had 

 been done by Clausius, and it appears from his investigations that whether we 

 suppose the molecules to be acted on by forces towards fixed centres or not, 

 the condition of equilibrium of exchange of energy, or in other words the 

 condition of equality of temperature of two bodies, is that the average kinetic 

 energy of translation of a single molecule is the same in both bodies. 



We may therefore define the temperature of a body as the average kinetic 

 energy of translation of one of its molecules multiplied into a constant which 

 is the same for all bodies. If we also define the total heat of the body as 

 the sum of the whole kinetic energy of its molecules, then the total heat 

 must be equal to the temperature multiplied into the number of molecules, 

 and by the ratio of the whole kinetic energy to the energy of translation, 

 and divided by the above constant. 



The kinetic theory of gases has therefore a great deal to say about what 

 Rankine and Clausius call the actual heat of a body, and if we suppose that 

 molecules never coalesce or split up, but remain constant in number, then we 

 may also assert, all experiments notwithstanding, that the real capacity for 

 heat (as defined by Clausius) is constant for the same substance in all conditions. 



* Hirst's translation, p. 230, German edition, 1864, p. 258, "wirklich vorhandene Warme, d.h. 

 die lebendige Kraft seiner Molecularbewegungen." 



VOL. IL 84 



