On the Elastic Force of the Vapor of Mercury. 291 



is at or near the freezing point of water. It follows, that althougb 

 the tension at such temperatures would be too small to be detected 

 by the column of mercury supported by it, a formula representing, 

 precisely, the law of tensions, would indicate the zero of tension 

 somewhere about the limit of which we have just spoken. The for- 

 mula e = (1 + 0.2527^) ^^•^''^ does not satisfy this condition, for when 



e=0, t= -5-0^' = -3.96 or 284° F.* It is thus evident that 



the tensions decrease more rapidly, with the decrease of tempera- 

 ture, within the range of temperature embraced by my experiments, 

 than would be shown by any expression which should also corres- 

 pond to the observations of Faraday. It is easy to see that the er- 

 rors of the formula would be increased, if the tensions were referred 

 to the temperatures obtained by an air thermometer, corrected for 

 the expansion of the glass, since my experiments refer to the com- 

 mon mercurial thermometer, which, at high temperatures, is in ad-, 

 vance of the air thermometer. 



It is by no means strange that this formula should fail to express 

 the law of tensions, through the whole extent of 648° F., between 

 the boiling point of mercury and the melting point of ice. It con- 

 tains but two arbitrary constants, to be determined by observation, 

 and, therefore, its use as an empirical formula is limited to a certain 

 range of temperature. The agreement of a similar formula with ob- 

 servations on the tension of the vapor of water, would seem to be 

 accidental. 



I have for the reasons developed in the preceding remarks, en- 

 deavored to represent my results by another formula, into which as 

 many arbitrary constants may be introduced, as are necessary to ex- 

 press the results of all observations. The formula, to which I refer 

 was first used by Laplace, in the Mecanique Celeste, to represent 

 the observations of Dalton on the tension of watery vapor : he found 

 it necessary to use but two terms of the formula. Three terms 

 were afterwards used by Biot, in his Traite de Physique, to express, 

 more accurately, the law of tensions of the vapor of water, between 

 32° and 212° F. This formula, calling A, the tension at the boiling 

 point, under atmospheric pressure ; e, the tension corresponding to 

 the temperature t, reckoned from the boiling point of the liquid, and 



* t= — == — 2.2, or 220° below the boiling point of mercury, that is, 140° C. 



