-1859] WRITINGS OF JOSEPH HENRY. 229 



visible vapor of a known weight and temperature. If we 

 calculate from the table B, (p. 225,) the amount of vapor which 

 at this temperature existed in this globe while its interior 

 was a vacuum, wo shall find it precisely the same as the 

 weight of that which the globe now contains when filled 

 with air. If, for example, the globe be a foot in capacity 

 and the small bulb contain 9"37 grains of water, the tempe- 

 rature at which the water disappears being 75°, by pass- 

 ing our e3''e horizontally along the table we shall find 

 under 75° the same number of grains. This experiment 

 conclusively proves that the same amount of vapor can exist 

 in a space already filled with air as in a vacuum. The 

 repulsive atoms of each however will be exerted against the 

 sides of the vessel, and the resulting pressure will be the sum 

 of the two; a fact which is proved by noting the height of 

 the column e, which indicates the elastic pressure of the air 

 in the globe before the vapor was admitted, and which, for 

 example, we may suppose to be equivalent to the weight of 

 20 inches of mercury. If we now open the stop-cock the 

 mercurial column will be depressed by the additional repul- 

 sion of the atoms of the vapor of water, and if the tempera- 

 ture be at 75° (as we have previously supposed) the depres- 

 sion will be 0"868 inches. 



The same result may be obtained by the following method, 

 which also gives us an independent means of determining 

 directly the amount of vapor which exists in the atmosphere 

 at a given time, and which may be employed for verifying 

 the results obtained by other means. Let a tight cask fur- 

 nished with a stop-cock near its lower part be entirely filled 

 with water, and let the small end of a tube which has been 

 drawn out in a spirit lamp be cemented into the vent-hole 

 above, so that no air can enter the cask except through the 

 tube. Let this tube be filled with coarsely powdered dry 

 chloride of calcium — a substance which has a great affinity 

 for moisture — and the upper end put in connection with an 

 open vessel containing air entirely saturated with moisture, 

 which can readily be effected by agitating a quantity of the 

 liquid in the vessel from which the air is drawn. Let the 



