20 



PROCEEDINGS OF THE NATIONAL MUSEUM 



with an expansion valve mounted at the top or inlet side of the coil. 

 The coil should have as many turns as space permits. A coU-within- 

 a-coil arrangement is most desirable (fig. 9). 



Figure 8. — Simple freeze-dry apparatus utilizing a condenser surface (.^= specimen 

 chamber; 5= commercial deep freeze; C^ condenser chamber; Z)= vacuum pump). 



A vacuum-tight drain should be installed at the bottom of the 

 condensing chamber to conveniently remove the condensation from 

 the chamber during defrosting. The condensing chamber should be 

 encased in an insulated box with openings in appropriate positions 

 for vapor lines and refrigeration tubing. 



An important requirement in any freeze-dry system is a valve (see 

 fig. 5) that will permit atmospheric air to enter the system in order 



Figure 9. — Condensing coil. 



that the chamber can be opened. This valve should be so located as 

 to insure that the condensing chamber is situated between the valve 

 and the specimen chamber, thereby assuring condensation of atmos- 

 pheric moisture on the condensing coil rather than on the specimen. 

 The valve must be vacuum tight when closed and need not be large; 

 a ){-inch intake is sufficient to shut down the system. It is recom- 



