504 ADDRESS DELIVERED BEFORE THE BRITISH ASSOCIATION 



ject of mechanical science as at present limited ; but it is not true 

 in a larger sense. In these, and such-like cases, the movement of 

 masses is transformed into molecular motion, and thus reappears as 

 heat, electricity, or chemical action ; and the amount of the 

 transformed action definitely corresponds to the mechanical force 

 which was apparently lost. 



In the cases just considered, mechanical action is converted 

 into molecular. But molecular actions of different kinds are 

 themselves in like manner interchangeable. Thus, when light is 

 absorbed, m viva is apparently lost ; but not to speak of phoxpho 

 rescence, in which the light absorbed, or a portion of it, is again 

 given out in all such cases, heat and chemical action are de- 

 veloped, and in amount corresponding to the loss. Hence the 

 apparent exceptions to the principle are in reality confirmations of 

 it ; and we learn that the quantity of force in nature is as un- 

 changeable as the quantity of matter. 



This, however, is not true of the quantity of available force. It 

 follows from Carnot's law, that heat can be converted into me- 

 chanical work only when it passes from a warmer to a colder 

 body. But the radiation and conduction, by which this is effected, 

 tend to bring about an equilibrium of temperature, and therefore to 

 annihilate mechanical force : and the same destruction of energy 

 is going forward in the other processes of nature. Thus, it follows 

 from the law of Carnot, as Professor Thomson has shown, that the 

 Universe tends to a state of eternal rest ; and that its store of 

 available force must be at length exhausted, unless replenished by 

 a new act of Creative Power. 



Mr. Rankine has attempted, in another method, to combine the 

 physical sciences into one system, by distinguishing the properties 

 which the various classes of physical phenomena possess in common, 

 and by taking for axioms propositions which comprehend their 

 laws. The principles thus obtained are applicable to all physical 

 change ; and they possess all the certainty of the facts from which 

 they are derived by induction. The subject-matter of the science 

 so constituted is energy, or the capacity to effect changes ; and its 

 fundamental principles are 1st, that all kinds of energy and work 

 are homogeneous or, in other words, that any kind of energy may 

 be made the means of performing any kind of work; and 2nd, that 

 the total energy of a substance cannot be altered by the mutual 

 action of its parts. From these principles the author has deduced 



