228 



WRITINGS OF JOSEPH HENRY. 



[1855- 



few moments in part converted into vapor while the mer- 

 cury was observed to be depressed. When the experiment 

 was repeated with different quantities of air above the mer- 

 curial column and at different temperatures, produced by 

 varying the heat of the water in the external tube, or which 

 would amount to the same thing, by varying the tempera- 

 ture of the room, the remarkable fact was discovered that 

 the depression of the mercurial column due to the introduc- 

 tion of the water was precisely the same at the same tem- 

 perature as when the experiment was made with a vacuum ; 

 for example, at the temperature of 60°, whatever might be 

 the elasticity of the air within the tube, the introduction of 

 the water always gave an additional depression of half an 

 inch. From this result the important fact is deduced that 

 the tension or elastic force of vapor in air is the same as that 

 of vapor in a vacuum; from which we might also infer that 

 the quantity of vapor which can exist in a given space al- 

 ready occupied with air is the same as that which can exist 

 in a vacuum at the same temperature. But this fact may be 

 directly proved by an independent experiment. 

 For this purpose let the globe a, Fig. 3, be filled 

 with air, while the small bulb placed within con- 

 tains a known quantity of water, and let the 

 globe thus filled be screwed to the top of the 

 barometer tube. If the apparatus be now par- 

 tially filled with mercury so as to leave the globe 

 nearly filled with air and the whole inverted with 

 its lower end in a basin of mercury, the mercury 

 will descend along the scale and will come to rest 

 at a certain division, which will indicate the elas- 

 tic force of the air in the globe ; if next the stop- 

 cock be shut and the small ball be broken by the 

 heat from a burning glass the contained water 

 will, in part at least, spring into vapor; and if we 

 gradually heat the globe until all the water dis- 

 appears and note the temperature at which this 

 takes place, the globe at this moment will be 

 filled with air at a known density and with in- 



FiQ. 3. 



