INTERMEDIATE FELDSPARS. 



39 



It is at once clear from these determinations that the solid phase 

 has the same composition as the liquid phase, so far as it is within the 

 power of chemical analysis to establish it. 



A-BiANj (Plates VI, VII, VIII, IX, X, XI). 

 In this feldspar we observe the same characteristics as in the two 

 preceding, but they are still less sharply marked. The viscosity is 

 greater, both solidification and melting take place more slowly, and 

 the undercooling is so persistent that the furnace must be cooled 

 slowly or the charge will come out wholly or partly vitreous. 



Ab,Anj,. 

 first preparation. 



Melting temperature, 1463. 



Here again we made an attempt to discover a possible difference in 

 composition in the first portions to crystallize out of the melt, this 

 time by optical means. We first cooled the charge so rapidly that 

 only a relatively small portion crystallized out in fine, reddish-brown 

 spherulites at the surface and near the wall of the crucible. Without 

 disturbing these, the crucible was then replaced in the furnace and 

 slowly reheated (about five hours) until the remaining vitreous mate- 

 rial had also become completely crystallized. Upon removing from 

 the furnace, the charge presented a singular appearance. The red- 

 dish-brown stars remained undisturbed, while the later crystals were 

 perfectly white. But though so different in appearance, the micro- 



