THE 

 LONDON, EDINBURGH and DUBLIN 



PHILOSOPHICAL MAGAZINE . 



AND 



JOURNAL OF SCIENCE. 



[THIRD SERIES.] 

 JANUARY 1849. 



I. On a 7}eto Empirical Formula for ascertaining the Tension 

 of Vapoiir of Water at any Temperature. By J. H. 

 Alexander, Esq.* 



THE formula which the following memoir is intended to 

 expose, is called ne'tx)\ because, to the best of my know- 

 ledge, it has never been used or suggested hitherto. It is 

 also rightly termed empirical^ in so far as it is not susceptible 

 of geometrical demonstration, but thus far only ; since in point 

 of fact, it was derived entirely from considerations d priori 

 and independent of any experiment or interpolation. Of 

 course, it was compared as soon as possible with the tempe- 

 rature corresponding to the ordinary atmospheric pressure ; 

 and after a satisfactory agreement had been found at this 

 point, the accord of the formula with observations at other 

 points, above and below,^ was regarded as neither accidental nor 

 surprising. The extent and nearness of this accord through 

 a range of experiment more extensive than has hitherto been 

 included in one and the same table, it is +he principal aim of 

 the present paper to exhibit, after having shown in few words 

 the reasonableness of the formula and its limits (or rather want 

 of limits) in application ; a comparison, then, of the errors ex- 

 isting and admitted in several of the experimental series of the 

 highest authority, with the differences developed at the same 

 epochs by the formula, will indicate the probabilities in favour 

 of the latter, and the nature and amount of its reliability. 



It is obvious that the pressure of vapour or steam must be 

 always in proportion to the absolute temperature at which it 

 is produced. But as this temperature is only observable re- 

 latively and upon an arbitrary scale, it is necessary, in order 

 to obtain anything like a measure of the quantity of heat ex- 

 isting, to use the ratio of the whole extent of the scale assumed 

 between the two epochs where the liquid changes its state 

 respectively, to that portion of it {i. e. the number of its de- 

 * From Silliman's Journal for Sept. 1848. 



PJiil. Mag. S. 3. Vol. 34. No. 226. Jan. 1849, B 



