THE PROGRESS OF PHYSICS. 139 



when heat passes into the air, but destruction of 

 energy is unknown. 



Energy is the power of doing work; work is the 

 net of producing a change of configuration in a sys- 

 tem in opposition to resistance; and the doctrine of 

 the conservation of energy is thus expressed by Clerk 

 Maxwell : " The total energy of any material system 

 is a quantity which can neither be increased nor 

 diminished by any action between the parts of the 

 system, though it may be transformed into any of the 

 forms of which energy is susceptible/' 



Dissipation of Energy. And to this doctrine of 

 conservation there has to be added the corollary, 

 which Sir William Thomson (Lord Kelvin) first 

 focussed into lucidity (1852) " the principle of dis- 

 sipation or degradation," which is " simply this, that 

 as every operation going on in nature involves a 

 transformation of energy, and every transformation 

 involves a certain amount of degradation (degraded 

 energy meaning energy less capable of being trans- 

 formed than before), energy is becoming less and 

 less transformable." * 



Foundation of the Doctrine of the Conservation of 

 Energy. Just as the doctrine of the indestructibility 

 of matter became stable with the perfecting of the 

 balance, so the doctrine of the conservation of energy 

 must be associated with the determination of the 

 mechanical equivalent of heat, with the experiments 

 of Rumford and Davy leading on to those of Colding 

 and Joule. At the same time, it should be borne in 

 mind that, according to Thomson and Tait, the prin- 

 ciple is clearly implied in Newton's scholium to his 

 third law of motion, that " if the action of an ex- 

 ternal agent is estimated by the product of its force 

 * P. G. Tait, Recent Advances (1876), pp. 145-6. 



