104 Day and Allen — Isomorphism and Thermal 



we were able to vary the beginning of solidification (crystalli- 

 zation) at will over a range of 250°, and (2) the temperature to 

 which the charge rose after the undercooled liquid had begun 

 to crystallize did not reach the melting point although once 



crystallization was induced only 

 10° below it in a furnace of con- 

 stant temperature. The rapidity 

 with which the crystallization and 

 the accompanying release of the 

 latent heat go on, depends in part 

 upon the rate of cooling and the 

 character of the disturbance which 

 has been applied, i. e., upon acci- 

 dental rather than characteristic 

 conditions. It thus happens that 

 the amount of the heat of fusion 

 and its slow rate of liberation in 

 the case of liquids which can be 

 greatly undercooled and become 

 very viscous, may be such as to 

 deprive it of its usual significance 

 as defining a solidifying point. 

 It is, of course, a consequence of 

 the phase rule that the solidifying 

 temperature of an undercooled 

 liquid is established, if only equi- 

 librium between solid and liquid 

 (and vapor) is reached before 

 complete solidification is accom- 

 plished, but equilibrium is not 

 necessarily attained during solidi- 

 fication, and the devices usually 

 employed (sowing with crystals, 

 agitating) are often totally inade- 

 quate to bring it about. The 

 temperature to which a crystalliz- 

 ing liquid rises after undercooling 

 is not necessarily constant or in 

 any way related to the melting 

 point, and is therefore not, in 

 general, entitled to be regarded as a physical constant. 



We then endeavored to ascertain whether the unstable 

 domain had a lower limit also. For this purpose we mixed a 

 quantity of the crystals with the glass and powdered them 

 together to about the fineness represented by a 150-mesh sieve 

 and heated them very slowly. In this condition the glass 

 proved to be very unstable and crystallized readily with a 

 rapid release of its latent heat at about 490°. Very slow heat- 























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Time (minutes). 



Fig. 5. 



