[ 197 ] 



XXIX. On the Expansion of Saturated Vapours. By W. J. 

 Macquorn Rankine, C.E., LL.D., F.R.SS.L. §• E., $c. 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, 



IN the Comptes Rendus of the French Academy of Sciences 

 for the 2nd of January, 1866, there appears a very inter- 

 esting and important communication by M. A. Cazin, in which 

 the author states the results of experiments which he has made 

 for the purpose of testing one of the conclusions deduced from 

 the laws of thermodynamics — viz., that when a vapour in the 

 state of dry saturation performs work by expansion in a non- 

 conducting cylinder, there is a certain critical temperature for 

 each vapour such that below that temperature the expansion is ac- 

 companied by partial liquefaction, and above it by superheating. 



M. Cazin mentions Professor Clausius and myself as having 

 simultaneously predicted in 1850 the liquefaction of vapours 

 under such circumstances; and M. G. A. Hirn as having shown, 

 in 1862, that for certain vapours at certain temperatures, and 

 in particular for ether, expansive working is accompanied by 

 superheating. He then refers to M. Dupre as having proved, 

 in 1864, that for each vapour there is a temperature below which 

 expansive working causes liquefaction, and above it superheat- 

 ing, as above described. 



Without detracting from the claim on the part of M. Dupre 

 to having arrived at that result by an independent investigation, 

 I wish to point out that the fact of there being such a critical 

 temperature for each fluid is clearly indicated in my paper a On 

 the Mechanical Action of Heat," published in the Transactions 

 of the Koyal Society of Edinburgh for 1850 (pages 170, 171), 

 and that an approximate formula for that temperature, with its 

 probable value for steam, is given by me in a paper " On Ther- 

 modynamics," published in the Philosophical Transactions for 

 1854 (pages 165, 166). The same formula with revised nume- 

 rical data is given in 'A Manual of the Steam-engine and other 

 Prime Movers/ page 386. 



The passages bearing on this subject in the paper of 1850 

 are as follows (Edinb. Trans, vol. xx. pp. 170, 171). The italics 

 are not in the original, being introduced now in order to point 

 out the particular parts of those passages which are now speci- 

 ally referred to : — 



" The apparent specific heat of a vapour at saturation is the 

 quantity of heat which unity of weight of that vapour receives 

 or gives out while its temperature is increased by one degree, 

 its volume being at the same time compressed so as to bring it 

 to the maximum pressure corresponding to the increased tem- 



Phil. Mag. S. 4. Vol. 31. No. 208. March 1866. P 



