230 WRITINGS OF JOSEPH HENRY. [1855- 



stop-cock be now opened and exactly a cubic foot of water 

 be drawn into a measured vessel; it is evident that precisely 

 a foot of air will enter the top of the cask through the tube 

 and between the interstices of the pieces of chloride of cal- 

 cium, the moisture will be absorbed, and its weight can be 

 accurately ascertained from the increase of weight of the 

 tube and its contents, which had previously been weighed 

 for that purpose. By this simple experiment as well as by the 

 one we have previously given we are enabled to conclusively 

 prove that the weight of vapor contained in the air in a 

 given space is the same as that which would exist at the 

 same temperature in a vacuum. To render the result of 

 this experiment absolutely perfect however a slight correc- 

 tion must be made on account of the expansion of the air and 

 the vapor due to the increased repulsive energy of the com- 

 pound over that of the air itself. This will be evident from 

 a due consideration of what follows. 



If into an extensible vessel, such as an India-rubber bag 

 filled with air, a little water be injected, the bag will be 

 suddenly expanded by the additional repulsive force of the 

 atoms of vapor. Previous to the introduction of the water, 

 the bag will be pressed equally on the outside and on the 

 inside ; on the former by the weight of the external atmos- 

 phere, and on the latter by the repulsive or elastic force of 

 the atoms of the inclosed air ; when the water is introduced 

 and a portion of it springs into vapor the elastic force of 

 the aqueous atoms must be added to that of the atoms of 

 the air, and the interior will then be pressed outward with 

 a force equal to the sum of the two repulsions. For example, 

 if the experiment be made at 60° and the air at its normal 

 weight, the outward pressure within the bag previous to the 

 introduction of the water will be equal to 30 inches of mer- 

 cury, but after the water is injected it will be 30 and a half 

 inches; hence expansion will take place and the bag will 

 be distended until by the separation of the interior atoms 

 the repulsion is so much weakened that the pressure with- 

 out and within will again be equalized. The amount of 

 the increase in bulk will be given by the following propor- 



