CHAPTER VIII. 



THE FORMS OF ENERGY. CONSERVATION OF ENERGY. 

 MECHANICAL EQUIVALENT OF HEAT. FIRST LAW OF 

 THERMODYNAMICS. 



Introductory Remarks The Various Forms of Energy The Identity of Energy 

 The Conservation or Constancy of Energy Statement of the Principle Mayer's 

 Calculation of the Mechanical Equivalent Joule's Researches Later Repetition 

 Experiments of Rowland of Miculescu of Reynolds and Morby of Griffiths 

 of Schuster and Gannon The First Law of Thermodynamics. 



Introductory Remarks. The investigation of the conditions under 

 which heat appears in a system, or disappears from it, leads us to 

 regard heat as one among various forms of energy, and when it appears 

 we find that some other form disappears, and vice versa. The investiga- 

 tion of the relation of heat to other forms of energy constitutes the 

 subject of Thermodynamics. In this chapter we shall set forth the 

 evidence which leads us to recognise various forms of energy, and to 

 adopt the principle known as the Conservation of Energy. This 

 principle, as applied to heat, is known as the First Law of Thermo- 

 dynamics. 



The study of mechanics leads to the recognition of two great principles 

 of conservation or constancy the conservation of mass and the constancy 

 of momentum in a given line. These principles have been recognised 

 from the time when Newton placed the science of dynamics on a firm 

 foundation. We have now to add a third great principle of conservation, 

 the Conservation of Energy. This principle was, naturally, only recog- 

 nised at a much later date, for its recognition depended on a much wider 

 knowledge of physical phenomena and their mutual relations than was 

 possible when the other principles were first enunciated. 



The fundamental idea that there is some identity underlying the 

 apparently different phenomena of mechanics, heat, light, and electricity 

 only assumed prominence at the beginning of the nineteenth century, 

 though it was sometimes vaguely perceived in earlier times. At first it was 

 expressed merely qualitatively, by saying that there was some mutual 

 relation between the various " forces of nature," or, as we should now 

 say, between the various forms of energy, so that one "force" was 

 convertible into other " forces." But as modes of measurement im- 

 proved, and numerical relations accumulated, it was gradually perceived 

 that quantitative relations held between the various correlated "forces," 

 and in 1845 we find Faraday saying : "I have long held an opinion, 

 almost amounting to conviction, in common, I believe, with many other 

 lovers of natural knowledge, that the various forms under which the 



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