118 Mr. W. J. M. Rankiue on the Mechanical Action of Heat. 



negative terms of this expression exceed the positive at all ordi- 

 naiy temperatures, so that the kind of apparent specific heat 

 now under consideration is a negative quantit)^ ; that is to say, 

 that if a given weight of vapour at saturation is increased in 

 temperature, and at the same time maintained by compression 

 at the maximum elasticity, the heat generated by the compression 

 is greater than that which is required to produce the elevation • 

 of temperature, and a surplus of heat is given out ; and on the 

 other hand, if vapour at saturation is allowed to expand, and at 

 the same time maintained at the tempei'atm'e of saturation, the 

 heat which disappears in producing the expansion is greater than 

 that set free by the fall of temperature, and the deficiency of 

 heat must be supplied from without, otherwise a portion of the 

 vapour roill be liquefied, in order to supply the heat necessary for 

 the expansion of the rest*. 



This circumstance is obviously of great importance in meteo- 

 rology, and in the theory of the steam-engine. There is as yet 

 no experimental proof of it. It is true that, in the working of 

 non- condensing engines, it has been found that the steam which 

 escapes is always at the temperature of saturation corresponding 

 to its pressure, and carries along with it a portion of water in 

 the liquid state ; but it is impossible to distinguish between the 

 water which has been liquefied by the expansion of the steam, 

 and that which has been carried over mechanically from the 

 boiler f. 



The calculation of the proportion of vapoiu- liquefied by a given 

 expansion, requires the knowledge of the latent heat of evapora- 

 tion, which forms the subject of the next section. 



* This conclusion is applicable only when the mechanical power pro- 

 duced by tlie expansion of the vapour is expended in moving another body ; 

 for example, the piston of an engine. When this power is expended in 

 producing currents in the vapour itself, the friction of those currents even- 

 tually reconverts the whole of the power into heat ; so that the vapour, 

 instead of being partially liquefied, is su])er-lieated, as has been shown theo- 

 retically by Professor William Thomson, and experimentally by Mr. Charles 

 W. Siemens. (Civil Engineer and Architect's Journal, September 1852.) 



t The experiments of Mr. Daniel Kinnear Clark, ou the expansive action 

 of steam in locomotive engines, described in his work " On Railway Ma- 

 chinery," show that a large amount of liquefaction generally takes place 

 during the expansion of the steam. It appeai-s, however, as Mr. Clark has 

 pointed out, that a considerable ])ortion of this liquefaction arises fi-om the 

 transference of he.at to the metal of the cylinder, and is followed by re-eva- 

 poration w hen the heat is transfcn'cd back, upon the pressure of the steam 

 falling below that due to the temperature of the metal. How much of the 

 liquefaction is due to this cause, and ho\\' much to the expansion of the 

 steam, it is impossible to determine, in the present imperfect state of our 

 knowledge of the bulk occupied by a given weight of steam at a given 

 pressure and temperature. 



