207 



should drop at a rate of some linndred degrees per second, 

 within the objects themselves. 



2. Vitrification. As a consequence of what has been 

 said on the position occupied by the vitreous state at the 

 lower end of the temperature scale and the impossibility 

 of passing from the crystalline to the vitreous state, the 

 only method of vitrifying a substance is to take it in the 

 liquid or gas state and cool it rapidly so as to skip over the 

 zone of crystallization temperatures in less time than is 

 necessary for the material to freeze. 



The ease with which an intermediate state can be skipped 

 over depends on the range of temperatures at which that 

 state obtains. The liquid state of CO:.., for example, is 

 easily avoided when solid C0_, is warmed up, on account 

 of the fact that the melting and the boiling point almost 

 coincide. 



The range of freezing temperatures, although large in 

 some substances under ordinary conditions, can become 

 narrower when such processes as subcooling occur. For 

 example, in a gel of which the freezing zone extends from 

 -2° to -12°, a subcooling to -7° will reduce this zone to 

 one half its normal size, that is, to a range of from - 7° to 

 -12°. 



Another factor of considerable import in the vitrifica- 

 tion of a substance is its crystallization velocity. It is 

 evident that when the crystals grow faster one must 

 traverse the crystallization zone more rapidly if one wants 

 to avoid crystallization. 



The essential point in the vitrification technique being 

 to overcome the velocity of formation of crystals, the main 

 problem is to secure a high cooling velocity. Practically 

 the only method used to cool a liquid or a solid body (for a 

 gas, it is different) is to bring that body in contact with 

 another at a lower temperature. The cooling velocity will 

 be higher Avhen the temperature of the cooling medium is 

 lower and when the contact is better. Liquid air (- 190°) 

 is appropriate for the vitrification of aqueous colloids, 

 aqueous solutions and protoplasm. Liquid hydrogen or 



