32 



ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS. 



7000 

 (775) 



6500 



and then introducing a few crystal fragments or starting the pound- 

 ing. Crystallization and release of the latent heat followed at once. 

 In fact over a range of some 250 immediately below the melting point 

 it proved to be within our power to precipitate the crystallization of 

 the undercooled mass entirely 

 at will. It was even possible 

 to cool the melted charge 

 quietly down to the tempera- 

 ture of the room and remove 

 it from the furnace as a clear 

 glass, then, on a subsequent 

 day, to reheat to some point in 

 this sensitive zone and pound 

 judiciously, when crystalliza- 

 tion would at once begin, 

 marked by the release of the 

 latent heat of the previous 

 fusion as before (fig. 6, a, b). 

 The accompanying curves 

 show the situation clearly. 

 Curves aa' and bb', fig. 7, were 

 obtained from charges of crys- 

 talline and vitreous borax, re- 

 spectively, of exactly equal 

 weight, which were cooled and 

 reheated in the same electric 

 furnace under like conditions. 

 The radiation from the furnace 

 for like temperature conditions 

 is practically the same, so 

 that the more rapid rate of 

 cooling and of reheating in the 

 crystalline charge indicates a 

 much smaller specific heat than 

 for the vitreous form. 



From the point of view of the 

 usual definition of the solidify- 

 ing point of a substance, a diffi- 

 culty confronts us here: (1) We were able to vary the beginning of 

 solidification (crystallization) 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 



6000 

 (680) 



O 



> 



o 



L. 

 O 



E 



C 5500 



o 



t_ 



3 



e 



a. 



e 



V 



I- 



5000 



4500 



4000 



(490) 



Time - I division = 10 minutes 



Fig. 6. Curves showing the release of the 

 heat of fusion at widely different tem- 

 peratures. 



