290 On the Elastic Force of the Vapor of Mercury. 



I observed, also, the tensions at temperatures below 446° F., but 

 conceive that at those temperatures the tension is so small and the 

 errors of observation so much increased, in reference to the tensions, 

 that the results are not to be relied upon. 



Having thus obtained, from experiment, the tensions of mercurial 

 vapor, corresponding to a considerable range of temperature, I en- 

 deavored to represent the result by some empirical formula, by the 

 aid of which I might obtain the approximate values of the tensions 

 at other temperatures, between that at which the tension begins to 

 be sensible, and that at which it is equivalent to the pressure of the 

 atmosphere. 



I tried first a formula which has been found to npply to the vapor 

 of water, namely, e=(l±at) m * in which e represents the tension of 

 the vapor, taking atmospheric pressure, or 29.94 inches of mercury 

 as unity; t the corresponding temperature, in degrees of the ther- 

 mometric scale, reckoned from the boiling point of the liquid, 100 

 degrees being taken as unity; a, a coefficient, to be determined, as 

 well as the exponent m, from experiment. This formula evidently 

 satisfies the condition, that at the boiling point of the liquid the ten- 

 sion is equal to the pressure of the atmosphere, since for t=0, we 

 have e=l, whatever may be the values of a and m. 



Determining a and m, in the formula just given, by the tensions 

 corresponding to the two extremes of temperature 446° and 554° 

 F., taking for unity 100 degrees, I find m=2.875, a=0.2527,f so 

 that the formula becomes e = ( 1 + 0.25272) 2 ' 875 . A comparison of 

 the tensions given by this formula, with those furnished by observa- 

 tion, shows that the formula may be considered as representing, very 

 nearly, the law of the tensions in terms of the temperature. 



There is an experiment which seems to show that this formula 

 does not truly express the law of the tension of mercurial vapor ; ai 

 all events, that it does not apply at tensions much lower than those 

 within the range of the experiments. 



We know that mercury emits, even at ordinary temperatures, a 

 vapor which is recognized by its deleterious action on the animal 

 economy, its chemical action on certain metals, &c. ; and from the 

 experiments of Faraday it appears that the limit to this evaporation 



* 



* This formula, as applied to the vapor of water, was printed erroneously in Vol 

 \IX, p. 182 of this Journal. It should have been. c=(l+0.7153/) 5 .— 7Van$. 

 t For the Cent, thermometer a = 0. 15 18, and the formula is £=(1+0.45480 2 8 7 5 



