4 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 119 



become equal to the rate at which they depart, and the water vapor 

 is then said to be in a state of equilibrium (fig. 1). The vapor pres- 

 sm'e at which this equilibrium occurs is referred to as equilibrium 

 vapor pressure. With the temperature of the chamber maintained 

 at — 10° C, the pressure indicated on its vacuum gauge will be 1.950 

 mm. Hg. This is the equilibrium vapor pressure (consequently the 

 vapor pressure) of water at —10° C. 



Water-vapor molecules may be removed from the area immediately 

 smTounding the ice in a vacuum chamber, thereby upsetting the 

 equilibrium and permitting more molecules to escape and allowing 

 fewer to retm-n. This principle applies to biological specimens as well 

 as to ice. 



The most effective method of continuously removing water-vapor 

 molecules from a specimen chamber is to create a lower vapor pressure 

 elsewhere. This can best be done by estabhshing a colder surface 

 (condenser) nearby. Water vapor will diffuse to a colder surface 

 and recondense to form new ice crystals. 



Figure 1. — Water- vapor equilib- 

 rium (^= refrigerated chamber; 

 5= vacuum gauge; C= cut-off 

 valve; Z)= vacuum pump). 



A refrigerated condenser serving as an efficient water pump is 

 employed for this purpose. As previously stated, when the tempera- 

 ture of a specimen chamber is —10° C, the vapor pressure of ice 

 within it is 1.950 mm. Hg. If the temperature of the nearby con- 

 densing surface is —40° C, the vapor pressure is 96 micron Hg., 

 creating a vapor-pressiu-e differential of 1.854 mm. Hg. Water 

 molecules will collect on the cold condenser surface. With this 



