-1859] WRITINGS OF JOSEPH HENRY. 231 



tion: as the pressure of 30 inches of mercury is to the pres- 

 sure of 30| inches, so is the original bulk of the India-rub- 

 ber bag to its bulk after the introduction of the vapor. 



From the preceding experiments and observations it is 

 evident that in free air the vapor exists as an independent atmos- 

 phere, being the same in weight and in tension as it would be in 

 a vacuum of the same extent and of the same temperature. That 

 the same amount of vapor can exist in a space filled with 

 air as in a vacuum at first sight appears paradoxical, but 

 when we consider that a cubic inch of water expanded into 

 steam at 212° occupies nearly 1,700 times the bulk which 

 it does in the form of water, also that air may be compressed 

 into a space many hundred times less than that of its ordi- 

 nary bulk, it is evident that the extent of the void spaces 

 is incomparably greater than the atoms themselves, and 

 consequently it is not difficult to conceive that the atoms 

 of the vapor have abundance of space in which to exist 

 between the atoms of air and the atoms of air between those 

 of vapor, Dalton announces this important truth by stat- 

 ing that air and vapor and almost all gases are vacuums 

 to each other. This enunciation is a true expression of the 

 state of diffusion which gases and vapors attain after the 

 lapse of a given time, but it does not truly express the 

 phenomena of the act of diffusion. In a perfect vacuum a 

 given space is filled with vapor almost instantaneously, or 

 with a rapidity which has not yet been estimated, but this 

 is not the same in a space already filled with air. In this 

 case, though the vapor ultimately diffuses itself through 

 the air as it would in a vacuum, yet time is required to pro- 

 duce this effect; the result is as if there were a mechanical or 

 some other obstruction to the free passage of vapor through 

 the different strata of air, and indeed it would appear from 

 the following experiments that a definite force similar to 

 that produced by a slight attraction or repulsion is offered 

 in the resistance of a given thickness of this medium: In 

 the laboratory of the Smithsonian Institution a glass tube 

 of about 3 feet in length, closed at its lower end, suspended 

 vertically, and containing about an inch of water, has re- 



