168 THE HISTORY OF SCIENCE 



expressed the hope of ascertaining the mechanical 

 equivalent of heat with the accuracy that its import- 

 ance for physical science demanded. He returned to 

 this question again and again. According to his final 

 result the quantity of heat required to raise one pound 

 of water in temperature by one degree Fahrenheit is 

 equivalent to the mechanical energy required to raise 

 772.55 pounds through a distance of one foot. Heat 

 was thus demonstrated to be a form of energy, the 

 relation being constant between it and mechanical 

 energy. Mechanical energy may be converted into 

 heat ; if heat disappears, some other form of energy, 

 equivalent in amount to the heat lost, must replace it. 

 The doctrine that a certain quantity of heat is always 

 equivalent to a certain amount of mechanical energy 

 is only a special case of the Law of the Conserva- 

 tion of Energy, first clearly enunciated by Joule and 

 Helmholtz in 1847, and generally regarded as the 

 most important scientific discovery of the nineteenth 

 century. 



Roscoe, referring to the two life-sized marble stat- 

 ues which face each other in the Manchester Town 

 Hall, says with pardonable pride : " Thus honor is 

 done to Manchester's two greatest sons to Dai- 

 ton, the founder of modern Chemistry and of the 

 Atomic Theory, and the discoverer of the laws of 

 chemical combining proportions ; to Joule, the founder 

 of modern Physics and the discoverer of the Law of 

 the Conservation of Energy." 



