20 



FIXATION PROCEDURES 



such as isopentane, rather than plunge the specimen directly 

 into liquid air or liquid nitrogen, since, if the specimen is di- 

 rectly in contact with liquid air or nitrogen, gas bubbles form 



-Vacuum flask 



■Bath at - 78°C 



Water-trapping 

 surface 



Speciman at 

 controlled temperature 



P,0, 



Fig. 1. Illustrating principle of new-t}^pe freeze-dryer. Former freeze- 

 dryers pumped the water off into a P2O5 trap. In the present type the 

 water is trapped on the walls of the evacuated tube, which are maintained 

 at —78° C. until the specimen is diy. Then on warming the tube the 

 water is returned by the P2O5 at the tube base. During the drying process 

 the specimen is warmed by a thermostatically controlled heater to a de- 

 fined temperature, usually —40° C. When the vacuum is maintained by 

 a two-stage rotary pump, drying requires about 2 days. If a diffusion 

 pump is used, backed by a rotary pump, the drying time can be reduced 

 to 6-10 hours for a specimen not more than 1 mm. thick. But there is 

 usually no advantage in reducing the drying time to this extent. 



around the specimen, due to vaporization of the liquid. As a 

 consequence, the specimen is insulated from the liquid air and 

 cools very much more slowly than when good thermal contact 

 is maintained between the cooling agent and the specimen. 

 Isopentane freezes at approximately —160°. When an optimal 



