288 On the Elastic Force of the Vapor of Mercury. 



The air was, previous to this experiment, and to the introduction 

 of the mercury, carefully dried, as well as the interior of the bulb 

 and tube, by placing the apparatus under a receiver with quick lime; 

 the mercury was then heated and introduced vi'ithout allowing any 

 communication between the tube and the exterior air. Without 

 these precautions the vapor of water, mixing with the air and mer- 

 curial vapor, would have vitiated all the results. 



To obtain the tension of mercurial vapor at temperatures above 

 212° F., the apparatus was placed in a vessel containing olive oil ; 

 the liquid covered the bulb, near which was placed a thermometer, 

 graduated beyond 672° F.* The bath was now heated gradually, 

 to this temperature, and the height of the mercury in the longer leg 

 of the siphon, corresponding to different temperatures, noted at inter- 

 vals. When the temperature of 572° F.-|- was attained, the apparatus 

 was allowed to cool slowly, noting the heights of the mercury, cor- 

 responding to the temperatures recorded in the ascending series of 

 observations. The errors produced by any difference in the tem- 

 perature of the mercurial vapor and the bath were thus eliminated, 

 by taking a mean of the observations corresponding to the same 

 temperature, in the ascending and descending series. 



In order to determine from these observations the height of the 

 column of mercury corresponding to the increase of tension derived 

 from the vapor of mercury in the bulb, it was necessary to correct 

 for the expansion of the air under the pressure to which the rise of 

 the mercury subjected it, and for the expansion of the mercury it- 

 self, in the longer leg of the siphon. The pressure, in addition to 

 that of the atmosphere, to which the contents of the bulb were ex- 

 posed, was the difference of level observed by the scale attached to 

 the longer leg added to the depression in the bulb, which latter could 

 be estimated with sufficient accuracy. This column was of course 

 to be reduced to the length which it would have measured at 32° F. 



In the memoir of which this is an extract, I give all the details of 

 these corrections and calculations, as applied to the apparatus used, 

 under the circumstances of the experiments ; I propose giving in this 

 place merely the resulting formula, by which the observations were 

 reduced. 



Call L the observed bulk of the air and vapor contained in the 

 bulb, at any temperature, expressed in divisions of the scale attached 



*SOO°C, t 300° C. 



