Properties of the Feldspars. 



103 



had taken on in melting (fig. 5, V). If it was subjected to the 

 jarring produced by the electric hammer on the furnace wall, 

 it cooled down a few degrees below the melting point and then 

 began to crystallize, the heat of fusion was set free and a rise 

 temperature or hump iinme 



m 



6500 



(680°) 



diately appeared upon the cool- 

 ing curve as shown in the figure 

 (fig. 3, b, e, d). Up to this point 

 the phenomenon differs but little 

 from the usual behavior of 

 liquids which undercool in solidi- 

 fying. We then varied the 

 experiment by first cooling quiet- 

 ly to about 100° below the melt- 

 ing point and then introducing 

 a few crystal fragments or start- 

 ing the pounding. Crystalliza- 

 tion and release of the latent 

 heat followed at once. In fact 

 over a range of some 250° im- 

 mediately below the melting 

 point it proved to be within our 

 power to precipitate the crystalli- 

 zation of the undercooled mass 

 entirely at will. It was even 

 possible to cool the melted charge 

 quietly down to the temperature 

 of the room and remove it from 

 the furnace as a clear glass, then, > 

 on a subsequent day, to reheat e 

 to some point in this sensitive^ 

 zone and pound judiciously,! 

 when crystallization would at § 

 once begin, marked by the^ 

 release of the latent heat of the 

 previous fusion as before (Hg. 4, 

 a and b). The accompanying curves 

 clearly. 



4500 



; 4000 



(490°) 



20 40 



Time (minutes). 



Fig. 



mo 



4. 



show the situation 



Curves aa' and W (fig. 5), were obtained from charges 



of crystalline and vitreous borax, respectively, 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 will also be 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 solidi- 

 fying point of a substance, a difficulty confronts us here: (1) 



