1885.] molecular energy on an absolute scale. 317 



the loss of heat, and in this way they may all be said to take 

 part in the work done by the engine supplied by them. They 

 all have their effect on the temperature of the supply on which 

 the availability of its energy essentially depends, and it may be 

 observed that in the category of forms of energy which thus con- 

 tribute to maintain the temperature of the supply must be in- 

 cluded the energy of dissociation, whether the dissociation consist 

 in the breaking up of the molecules into parts which are homo- 

 geneous, as happens with sulphur vapour and many other vapours 

 at high temperatures, or whether it is dissociation of chemically 

 diverse kinds of material. 



Now considering that all these several forms of energy con- 

 tribute to the temperature of a body it seems hardly possible to 

 avoid the conclusion that they are all subject to the thermo- 

 dynamic laws and that the conclusions drawn from those laws are 

 all applicable, mutatis mutandis, to them as well as to heat. Many 

 authors appear to me to have made tacitly some such assumption, 

 and I have myself done so avowedly in considering the influence 

 of dissipation of energy in regard to Chemical Equilibrium \ It 

 is possible however to follow out the reasoning with regard to 

 each form of kinetic molecular energy in lines parallel to those 

 employed by writers on thermo-clynamics. 



The second law of thermo-dynamics is a particular case of 

 the more general law of dissipation of energy. That law may be 

 stated as follows : 



Every change which takes place spontaneously in the form 

 or distribution of energy in any given portion of matter without 

 accession of energy from without reduces the availability of the 

 energy of that matter to do mechanical work. 



No machine self-acting can then convert energy from a form 

 in which it is less available into one in which it is more 

 available. 



It follows then on Carnot's principle that if we have a re- 

 versible engine working between a supply of energy at one degree 

 of availability and a sink at some ve^ slightly lower degree of 

 availability, the fraction of each unit of energy derived from the 

 supply which can be converted by the engine into mechanical 

 work will depend only on the degree of availability of the supply 

 and on the difference of degrees of availability between the supply 

 and sink. 



The expression degree of availability is an awkward one and 

 not always appropriate so that I prefer to use " potential " instead 

 of it. The fundamental notion connected with equality of potential 



1 Chemical Equilibrium the result of the dissipation of Enerqy. Cambridge, 

 1885. Deighton, Bell and Co. 



22 2 



