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XLIII. On Pressures of Saturated Vapours. 

 By W. J. Macquorn Rankine, C.E., F.R.S. 



To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, 



IN consequence of some remarks which have been addressed 

 to me respecting my letter on the above subject in the 

 Number of your Magazine for October, I think it desirable to 

 explain, that the formula 



logP=A-^--^- (2) 



is in no way deduced from the formula 



logP = a— i.logT , (3) 



and that the coefficients A, B, C are not functions of a, b, c. In 

 fact, the formula (2) was suggested as probable by the hypothesis 

 of an attraction between the liquid and its vapour, as explained 

 in the Numbers of your Magazine for December 1851 and Decem- 

 ber 1854; and A, B, C are part of an indefinite series of con- 

 stants determined empirically, so as to agree with M. Regnault's 

 experiments; while (3) is the result of an independent hypo- 

 thesis as to a condition of saturated vapour, which may, as I 

 have already stated, prove to be purely ideal, and respecting 

 which, therefore, I refrain from entering into detailed investiga- 

 tions until we possess exact data as to the densities of vapours. 

 The formula of Professor Holtzmann, 



logP = a , 



T 



appears to be nearly the same with that of Roche, and may be 

 held to constitute the first approximation towards the law of the 

 maximum pressure of vapours. 



I have the honour to be. Gentlemen, 



Your most obedient Servant, 



W. J. Macquorn Bankine. 

 Glasgow, October 1, 1855. 



P.S. Since the above remarks were written. Sir John Lubbock 

 has done me the favour to send me a table showing the differ- 

 ences between the results of his formula (1) and those of the 

 experiments of M. Regnault on steam ; the three constants in 

 the formula having been determined from the same data with 

 those in the formula published by me in 1849, viz. the pressures 

 of steam at 26°-86, 100°, and 220° Centigrade. 



These diflferences are, on the whole, greater than those result- 



