CHAPTEE I 

 FOUNDATIONS OF THE HYPOTHESIS 



1. The Nature of Heat 



THE proposition that heat is not a substance, but a form 

 of energy, is no longer considered a hypothesis, but the ex- 

 pression of a certainly proved fact. Experiment shows that 

 heat is generated when mechanical motion is destr^|P^by 

 friction or otherwise ; and, on the other hand, the perform- 

 ance of work by engines that are driven by heat is taken as 

 a proof that heat can be converted into ordinary mechanical 

 energy. 



From these and other observations it follows that heat 

 is of the same nature as mechanical work, kinetic energy 

 of visible motion, and all other forms in which energy shows 

 itself in nature. Measurement further proves that the same 

 amount of heat always corresponds .to a given expenditure 

 of mechanical energy. Heat therefore undoubtedly forms 

 one of those indestructible magnitudes which we class as 

 energy ; or, in other words, heat is energy. 



But in every branch of physics generally, as in mechanics, 

 there are two species of energy, which may be distinguished 

 as potential 1 and kinetic, 2 and in heat both species are 

 recognised latent heat, for instance, is for the most part 



1 The term^ potential energy was applied by Ran kin e (Phil. Mag. [4], v. 

 1853, p. 106) to the magnitude called vis mortua by Leibniz (Acta Erud* 

 Lips. 1695, p. 149; collected works, Gerhardt's ed. vi. 1860, p. 238), and 

 later called Spannkraft byHelmholtz. 



2 The term kinetic energy was first employed by Thomson (Lord Kelvin) 

 and Tait (Treatise on Natural Philosophy, Oxford 1867) in place of actual 

 energy, E a n k i n e ' s name for half of the magnitude called vis viva by L e i b n i z . 

 This was termed simply energy by Thomas Young (Lectures on Nat. PhiL 

 lect. viii. London 1808, ,p. 79 ; new ed. 1845, p. 59). 



B 2 



