“5m 
water is placed in a container covered with insulation and the air 
completely removed, the system comes finally to a constant tempera- 
ture and the vapor pressure reaches a value which depends on this 
temperature alone. This value of vapor pressure for various tem- 
peratures is given in Table 1. The unit of pressure employed is the 
millibar, defined as 1000 dynes per square centimeter. In turn, 
1000 mb is a little less than one atmosphere. 
In another experiment suppose the pan is covered and placed in 
the container with dry air, and this time the container is not in- 
sulated but placed in a constant temperature bath. If the pressur 
in the container is measured after its temperature comes to equilib- 
rium, then the cover removed from the pan and the pressure measured 
after equilibrium is again reached, it is found that the pressure in- 
ereases by an amount eaual to the vavor pressure attained at the same 
temperature in the first experiment. Furthermore the amount of water 
evaporated is the same in each casee So in each case the vapor pres= 
sure and the density of water vapor is the same. 
In the final state of either experiment the vapor is said to be 
saturated, The vapor pressure attained is thus independant of the 
presence of air and depends only on the temperature, and this function 
In the first experiment the evaporation is almost instantaneous 5 
it is necessary to wait for complete equilibrium only in order that 
temperature equalize completely throughout vanor and liacuid water. ° 
In the second experiment the rate of evaporation is slow, because the 
vapor diffuses slowly through the air. Even in the second experiment, 
however, the adjustment immediately at the water surface is instan-= 
taneous, so that at the interface there is saturation and air and 
water have the same temperature. 
