FIXATION PROCEDURES 21 



result is required, it is sometimes desirable to achieve a some- 

 what lower temperature. This can be done by using a eutectic 

 mixture of propane and isopentane, or propane and butane, in- 

 stead of isopentane. 



The drying chamber which we have developed has an easily 

 demountable head, attached to which are the racks which carry 

 the specimens. The procedure which we use is to surround the 

 drying chamber with an acetone-solid CO2 mixture which main- 

 tains the drying unit at approximately —78° C. The demount- 

 able head is placed with the specimen racks in liquid nitrogen. 

 When the specimen has been frozen in the cooled pentane, the 

 racks are lifted out of the liquid nitrogen, the specimen placed 

 on the racks, and the unit placed in the drying chamber and 

 evacuated. From the time the specimen is lifted out of the 

 pentane bath to the time when it is placed in the drying chamber 

 at —78°, only a few seconds elapse. 



The general procedure which we use is then to dry the 

 material for two or three days. The walls of the drying chamber 

 are maintained at —78°, and the drying chamber is arranged 

 so that the racks containing the specimens are electrically heated 

 to a defined temperature, variable between —60° C. and —10° C. 

 as required. Under these conditions the water vapor from the 

 specimen sublimes from the specimen to the walls of the drying 

 tube. This involves a very short vapor. path. Consequently, 

 the pumping unit has no load placed upon it. It is not necessary 

 for the water vapor to be pumped over into a water trap as has 

 commonly been the case in previous drying units. The pump 

 is used merely to maintain the basic low pressure, and the 

 water vapor sublimes from the specimen to the sides of the dry- 

 ing chamber. When the specimen has been sufficiently dried, 

 the temperature of the drying chamber is allowed to rise to 

 room temperature. When this occurs, the water vapor rapidly 

 sublimes into a phosphorous pentoxide trap. If desired, this 

 latter stage can be avoided by having a phosphorous pentoxide 

 component in the drying chamber itself. But we have encoun- 

 tered no difficulty with this procedure of subliming the water 

 from the specimen to a more highly cooled surface a short 

 distance away, followed by a rapid distillation of this water to 

 a trap, carried out at room temperature when the specimen 

 is already dried. The apparatus has been constructed so that 



