THE PROGRESS OF SCIENCE FROM 1836 TO 1886. 511 



merit of that profound fundamental physical principle, the conservation 

 of energy. Even before the beginning of our half-century, Davy and 

 Rumford (especially the latter) had caught faint glimpses of the com- 

 ing truth in this direction. They recognized that heat was a mode of 

 motion, and Rumford went so far as to observe that the energy gener- 

 ated by a given amount of hay burned in an engine might be measured 

 against the energy generated by the same amount of hay consumed by 

 horses. But to Dr. Joule, of Manchester, in our own time, is due the 

 first great onward movement, in the discovery and determination of 

 the mechanical equivalent of heat. Joule's numerous experiments on 

 the exact relation between heat and mechanical energy resulted in the 

 establishment of a formula of equivalence in terms of kilogrammetres 

 necessary to raise by one degree centigrade the temperature of one 

 kilogramme of water. More properly put, he showed that the energy 

 required to raise a weight of one hundred pounds through one foot 

 was equivalent to the amount required to raise a certain fixed quantity 

 of water through one degree in temperature. 



Starting from this settled point, it soon became clear to physical 

 thinkers that every species of energy was more or less readily converti- 

 ble into every other, and that an exact numerical equivalence existed 

 between them. This principle, which first clearly emerged into the 

 consciousness of physicists about the middle decades of the present 

 century, was originally known under the name of " Persistence of 

 Force," in which form Grove's well-known little treatise helped large- 

 ly to popularize its acceptance. But, as time went on, the underlying 

 distinction between force and energy came to be more definitely real- 

 ized, and the phrase conservation of energy began to supersede the 

 older and erroneous terminology. The realization of the varying 

 nature of energy as potential and kinetic helped in the transformation 

 of the prime concept. At last, under the hands of Clausius, Helm- 

 holtz, Mayer, Clerk Maxwell, Tait, and Balfour Stewart, the doctrine 

 assumed its modern form that all energies are mutually convertible, 

 and that the sum-total of energy, potential and kinetic, is a constant 

 quantity throughout the cosmos. 



The practical applications of the doctrine of energy are as yet only 

 in their infancy. The whole mass of theoretical science has to be 

 re-written in accordance with this new and fundamental law. - The 

 whole field of applied science has to be developed and enlarged by the 

 light of this pregnant and universal principle. Its implications are 

 all-pervading. In astronomy it has profoundly affected all our con- 

 ceptions as to the sun's heat, the orbits of planets, the nature of me- 

 teors, the past, present, and future of the universe. In biology it has 

 taught us to envisage the plant mainly as a machine in which kinetic 

 energy is being transformed into potential ; the animal mainly as a 

 machine in which potential energy is being transformed back again 

 into kinetic. In mechanics and the mechanical arts it has produced 



