390 Mr. W. J. ]\Iacquorn Rankine on the Science of Energetics. 



or of the opposite sign ; that is, according as X is of the same sign with 

 dx, or of the opposite sign. 



It is to be observed, that the states of substances comprehended^under 

 the term actual energy, may possess the characteristics of potential energy 

 also ; that is to say, may be accompanied by a tendency or effort to vary 

 relative accidents ; as heat, in an elastic fluid, is accompanied by a ten- 

 dency to expand ; that is, an effort to increase the volume of the recep- 

 tacle containing the fluid. 



The states to which the term, potential energy, \s especially applied, 

 are those which are solely due to mutual actions. 



To put a substance into a state of energy, or to increase its energy, is 

 obviously a hind of work. 



IX. First Axiom. 



All kinds of Work and Energy are Homogeneous. 



This axiom means, that any kind of energy may be made the means of 

 performing any kind of work. It is a fact arrived at by induction from 

 experiment and observation, and its establishment is more especially due 

 to the experiments of M. Joule. 



This axiom leads, in many respects, to the same consequences with the 

 hypothesis that all those kinds of energy which are not sensibly the re- 

 sults of motion and motive force are the results of occult modifications of 

 motion and motive force. 



But the axiom differs from the hypothesis in this, that the axiom is 

 simply the generalized allegation of the facts proved by experience, while 

 the hypothesis involves conjectures as to objects and phenomena which 

 never can be subjected to observation. 



It is the truth of this axiom which renders a science of energetics 

 possible. 



The efforts and passive accidents to which the branches of physics 

 relate are varied and heterogeneous ; but they are all connected with 

 energy, a uniform species of quantity, which pervades every branch of 

 physics. 



This axiom is also equivalent to saying, that energy is transformable 

 and transferable (an allegation which, in the previous paper referred to, 

 was included in the definition of energy) ; for, to transform energy, means 

 to employ energy depending on accidents of one kind, in putting a sub- 

 stance into a state of energy depending on accidents of another kind ; and 

 to transfer energy, means to employ the energy of one substance in 

 putting another substance into a state of energy, both of which are kinds 

 of work, and may, according to the axiom, be performed by means of any 

 kind of energy. 



