-1859] 



WRITINGS OF JOSEPH HENRY. 



217 



uated scale for measuring the height of the column is 

 denoted by b. The larger tube around the barometer tube 

 to contain the water of different temperatures is denoted 

 by d. A thermometer e is inserted at its upper end by 

 which to ascertain the temperature of the enclosed water 

 and, consequently, that of the vapor within the barometer. 



With this simple contrivance Dr. Dalton made a series of 

 experiments to determine the repulsion of the atoms of 

 steam ; or in other words, the elastic force of aqueous vapor, 

 corresponding to the different degrees of Fahrenheit's scale 

 from zero up to the boiling point. To facilitate the opera- 

 tions and to allow for any changes that might take place 

 in the pressure of the atmosphere during the continuance 

 of the experiment, another tube was placed beside the first 

 in the same basin, and the descent of the mercurial column 

 of the first tube estimated from the top of that in the second, 

 which to render the measure more accurate ma}^ be effected 

 by means of a small telescope, sliding on a graduated rod, 

 and movable in a horizontal plane. 



By placing water of a given temperature within the outer 

 tube and gradually cooling it after each observation, and 

 finally filling the same tube with freezing mixtures, a table 

 similar to the following was constructed. Dalton's experi- 

 ments however have been repeated with additional precau- 

 tions by other scientists, and particularly by M. Regnault, 

 from whose work the annexed table has been compiled. 



A — Elastic force of aqueous vapoi', in English inches of mercury. 



