A CENTURY'S PROGRESS IN PHYSICS 345 



a machine capable of perpetual motion, even though the 

 "imponderable agents " of electricity, galvanism and 

 magnetism be utilized. 



Thermodynamics. The importance of the principle of 

 conservation of energy lies in the fact that it unites under 

 one rule such diverse phenomena as gravitation, electro- 

 magnetism, heat and chemical action. Another principle 

 as universal in its scope, although depending upon the 

 coarseness of human observations for its validity rather 

 than upon the immutable laws of nature, was fore- 

 shadowed even before the first law of thermodynamics, 

 or principle of conservation of energy, was clearly 

 recognized. This second law was the consequence of 

 efforts to improve the efficiency of heat engines. In 1824 

 Carnot introduced the conception of cyclic operations 

 into the theory of such engines. Assuming the impos- 

 sibility of perpetual motion, he showed that no engine can 

 have an efficiency greater than that of a reversible 

 engine. Finally Clausius expressed concisely the princi- 

 ple toward which Carnot 's work had been leading, when 

 he asserted that "it is impossible for a self-acting 

 machine, unaided by any external agency, to convey heat 

 from one body to another at a higher temperature. " 

 Kelvin's formulation of the same law states that "it is 

 impossible, by means of inanimate material agency, to 

 derive mechanical effect from any portion of matter by 

 cooling it below the temperature of the coldest of the 

 surrounding objects. ' ' 



The consequences of the second law were rapidly 

 developed by Kelvin, Clausius, Rankine, Barnard (16, 

 218, 1853, et seq.) and others. Kelvin introduced the 

 thermodynamic scale of temperature, which he showed 

 to be independent of such properties of matter as con- 

 dition the size of the degree indicated by the mercury 

 thermometer. This scale, which is equivalent to that of 

 the ideal gas thermometer, was used subsequently by 

 Rowland in his exhaustive determination of the mechan- 

 ical equivalent of heat by an improved form of Joule's 

 method. He found different values for different ranges 

 in temperature, showing that the specific heat of water 

 is by no means constant. Since then electrical methods 



