PHYSICS OF NINETEENTH CENTURY 177 



of a gas, which was extended and made more 

 rigorous by James Clerk Maxwell. On this theory, 

 most of the physical relations of a gas can be explained 

 directly in terms of simple movements of the hypo- 

 thetical molecules of which it is imagined to be 

 composed. 



Joule's definite experimental result that work and 

 heat were equivalent gave point and power to a more 

 general conception which was then arising, known as 

 the " correlation of forces," and developed it into the 

 important physical principle of the conservation of 

 energy. Energy, as a definite physical conception, 

 was new to science. The idea which underlay it had 

 previously been expressed by an inaccurate and 

 confusing extension of the meaning of the word force. 

 Energy may be defined as the power of doing work, 

 and, if the conversion be complete, may be measured 

 by the work done. 



Joule's experiments showed that, in one definite 

 case, the total amount of energy in a system is constant, 

 the quantity lost as work reappearing as heat. General 

 evidence led to the extension of this result to other 

 changes, where, for instance, mechanical energy is 

 converted into electrical energy, and then perhaps into 

 chemical energy. All known facts are consistent 

 with the statement that, in all cases in present con- 

 ditions, the total energy of an isolated system is 

 constant in amount. 



The principle of the conservation of energy, thus 

 established, is comparable with the long -known 

 principle of the conservation of mass. Newtonian 

 dynamics are founded on the recognition that there is 

 a quantity, for convenience called the mass of a body, 



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