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MACROMOLECULAR COMPLE'XES 



Fig. 1. Equipment for rapid freezing of biological specimens in liquid 

 helium II with attached gas thermometer, high-speed camera, and microscope 

 for direct observation of specimens within helium Dewar at —272 C. Arrow 

 indicates specimen stage for introducing fresh or glycerinated tissues into 

 liquid helium II without breaking vacuum. (Installed at the Cryogenic Engi- 

 neering Laboratory, M. I. T., in cooperation with Professor Samuel C. Collins.) 



changes from occurring. However, very rapid cooling leads to 

 severe distortion and cracking of larger tissue sections, and the opti- 

 mum cooling velocity must therefore be determined for each type 

 of specimen. 



A simplified version of this equipment has proved to be verv 

 useful for achieving rapid and ultrarapid cooling of tissues with 

 other types of refrigerants. Thus, if liquid nitrogen is cooled close 

 to its fusion temperature (63.15° K) by evacuating the Dewar, small 

 tissue specimens can be introduced through the attached vacuum 

 stage, and appear to freeze more rapidlv in this quiescent fluid than 

 in the boiling liquid nitrogen at atmospheric pressure. Since the 

 use of liquid nitrogen under these conditions yields excellent tissue 

 preservation, it can be recommended as an inexpensive alternative 

 method or training procedure for helium cryofixation. 



