8 MICROSCOPIC TECHNIQUES 



low value, the pressure gradient between F and / disappears. How- 

 ever, since water may diffuse out, it has been found desirable to 

 continue the pumping for a day or more after the pressures at 

 F and / are coincident and remain so. 



The ionization gauges used, of the type described by Montgomery 

 and Montgomery ( 1938) , are No. 47 radio tubes of Radio Corpora- 

 tion of America, which are sealed to the vacuum system with black 

 vacuum wax, care being taken to avoid knocking off the filament 

 coating during the sealing. A power pack is employed consisting 

 of a half-wave rectifier, filter system, and power transformer. An 

 additional filament transformer is included to supply the filament 

 of the second gauge. In order that the same meters and galva- 

 nometer may be used for both gauges a double-throw triple-pole 

 switch is employed. For pressures less than 3 X 10"^ mm. of mer- 

 cury a student type of wall galvanometer is used to measure positive 

 ion current in the gauges; this current is directly proportional to 

 the pressure. For higher pressures a microammeter with a range 

 of 0-50 may be used. Since the usual calibration constant (1 

 yuamp. for 7 X lO"*' mm. of mercury) is for air, a different constant 

 would apply in the presence of water vapor, hence only relative 

 pressures are obtained. 



PROCEDURE 



1. Place paraffin in apparatus; melt and degas it in vacuo by 

 means of the mechanical pump alone. 



2. Let the paraffin solidify and raise the cooling Dewar flask 

 around the drying chamber. When equilibrium is attained the 

 system is ready for the tissue. 



3. Either freeze the tissues in liquid air or, preferably, in isopen- 

 tane at liquid air or liquid nitrogen temperatures. Violently agitate 

 the isopentane to speed the heat extraction. 



4. Place frozen tissue in the copper gauze basket and transfer 

 immediately to drying chamber of apparatus. 



5. Start the mechanical pump at once; then start the diffusion 

 pumps and run for 12 hr. before taking pressure readings. The 

 gauge filaments must be heated and gas allowed to escape for 

 several hours before reliable readings are possible. After the read- 

 ings of both gauges are equal, continue pumping for some time 

 depending on the size, shape, and character of the tissue. No rule 



