216 



WRITINGS OP JOSEPH HENRY. 



[1855- 



elastic force or repulsion of the atoms of the aqueous vapor 

 will be evident when we consider that if we remove the vapor 

 the column will rise to 30 inches, and will then be exactly 

 in equilibrium with the pressure of the external atmosphere, 

 the two being in exact balance ; but if after the introduction 

 of the vapor the column is reduced half an inch in height, 

 it is plain that the force which produces this effect must be 

 just equal to the weight of this amount of mercury. 



Dr. Dalton next diminished the length of this vacuum by 

 plunging the lower end of the tube deeper into the basin of 

 mercury, and thereby causing the upper end of the column 

 to be projected farther into the tube; but this produced no 

 difference in the height of the column, the top of which was 

 still depressed to half an inch below the normal height of 30 

 inches. From this experiment we infer that the repulsion 

 of the atoms of vapor cannot, like that of the atoms of air, be 

 increased by external pressure; for when we attempt to 

 coerce them into a smaller space by ex- 

 ternal pressure, a portion of them is con- 

 verted into water, and the atoms which 

 remain in the aeriform condition exert 

 the same amount of pressure as before. 



Dr. Dalton next increased the tempe- 

 rature by surrounding the tube contain- 

 ing the mercurial column with a larger 

 tube filled in succession with water of 

 different temperatures; this produced for 

 each temperature a difference in the 

 depression of the height of the column; 

 and when the water was at the tempera- 

 ture of 100° the depression instead of 

 being half an inch was almost precisely 

 three times as much. 



Fig. 1 represents the apparatus em- 

 ployed by Dr. Dalton, in which a is the 

 barometer tube filled with mercur}^ to 

 the height of/, and its lower end plunged 

 Fig- 1- into the basin of mercury c. The grad- 



