140 PROGRESS OF SCIENCE IN THE CENTURY. 



into its velocity, and the reaction of the resistance in 

 the same way by the product of the velocity of each 

 part of the system into the resisting force, arising 

 from friction, cohesion, weight, and acceleration, the 

 action and reaction will be equal to one another, what- 

 ever be the nature and motion of the system." 



We have placed the doctrine of the conservation of 

 energy before the dynamical theory of heat because 

 many discoveries were pointing towards the great 

 conclusion of the transformability and conservation of 

 energy, before Joule's measurement of the mechanical 

 equivalent of heat made the vaguely foreseen conclu- 

 sion an established doctrine. ISTone the less, however, 

 would we emphasise that the establishment of the 

 general doctrine dates from Joule's success as a 

 measurer of the relation between heat and mechanical 

 work in 1843. 



For it was then that one of the greatest scientific 

 steps of the century was made. " Clear and unques- 

 tionable experimental proof was given of the fact that 

 there is a definite relation between mechanical work 

 and heat ; that so much work always gives rise, under 

 the same conditions, to so much heat, and so much 

 heat to so much mechanical work. Thus originated 

 the mechanical theory of heat, which became the start- 

 ing point of the modem doctrine of the conservation 

 of energy. Molar motion had appeared to be destroyed 

 by friction. It was proved that no destruction took 

 place, but that an exact equivalent of the energy of the 

 lost molar motion appears as that of the molecular 

 motion, or motion of the smallest particles of a body, 

 which constitutes heat. The loss of the masses is the 

 gain of their particles." * 



* T. H. Huxley, Essay on ♦' The Progress of Science" (1887), 

 in Method and Remits, 1894, pp. 85-86. 



