On the Elastic Force of the Vapor of Mercury. 287 



through which to communicate heat, a method which would have 

 been scarcely practicable. In the mode of experiment just referred 

 to, the only effect of the vacuum is, that the vapor rises as soon as 

 formed. If air be present, the vapor will ultimately rise in the same 

 quantity as if not subjected to pressure, and its elasticity, added to 

 that of the air within, will depress the column of mercury; the amount 

 of the depression being measured, and correction being made for the 

 expansion of the air by heat, the tension of the vapor, at the tem- 

 perature of observation, becomes known. 



The apparatus which I used in determining the tension of mercu- 

 rial vapor, consisted of a glass siphon tube, the longer leg of which 

 was open, and the shorter terminated by a bulb. The shorter leg of 

 the siphon, and about two thirds of the bulb, were filled with mercu- 

 ry, which rose to nearly the same level in the two legs ; so that the 

 air confined in the bulb was nearly of atmospheric density. When 

 most expanded by heat, and with the additional elasticity of the 

 mercurial vapor, the air hardly filled the whole of the bulb, and, 

 therefore, never reached the leg. The surface of mercury, on 

 which the air rested, was, therefore, that of a section of the bulb, 

 and was always considerable ; this arrangement insured the satura- 

 tion of the space, above the mercury, with mercurial vapor, at the 

 different temperatures. To the longer leg of the siphon a metallic 

 scale was attached, the divisions being in twenty fifths (.04) of an 

 inch.* By this scale the increase of tension of the air in the bulb, 

 by heat, and by the tension of the mercurial vapor, was readily 

 measured. To ascertain the bulk occupied by the air in the bulb, 

 the apparatus was plunged into boiling water, at the temperature of 

 which the tension of mercurial vapor is known not to be apprecia- 

 ble. From the rise of the mercury, as shown by the scale, and the 

 law of expansion of air by heat, after correcting for the expansion of 

 the mercury and for the pressure due to the difference of level of 

 the mercury in the two legs, the number of divisions on the scale 

 corresponding to the whole bulk of the air in the apparatus at a tem- 

 perature of 32° F. was found. f This result was in accordance 

 with the approximate estimate, obtained by measurement of the bulb 

 and tube. 



* Millimetres. 



t A further correction was necessary for the expansion of the glass, to render (he 

 result rigidly exact. The apparent expansion of air contained in glass is substituted 

 for its real expansion throughout the inquiry. — Trans. » 



