INTERMEDIATE FELDSPARS. 



41 



AboANj. 

 To effect the complete crystallization of this substance, it is best to 

 reduce it to a fine powder and heat very slowly, holding the temper- 

 ature for many days at ioo to 200 below the melting point. When 

 thoroughly crystallized, it has a melting temperature which is deter- 

 minable with reasonable certainty, but neither this nor any of its 

 thermal phenomena approach the more calcic feldspars in sharpness. 

 For this reason a considerably greater variation will be noticed in the 

 melting points tabulated below: 



Ab 2 An!. 

 first preparation. 



Date. 



Dec. io, 1903 

 Dec. 15, 1903 

 Dec. 16, 1903 



Do. 

 Jan. 18, 1904 

 Feb. 29, 1904 



Do. 



Electromotive 

 force in MV. 



13,726 



13,887 



13,969 

 13,728 



13,967 

 13,812 



13,854 



Tempera- 

 ature. 



I362 c 



1374 

 1381 

 1362 

 1376 



1363 

 1366 



Remarks. 



Very rapid heating. 



Poor. 



Covered. 



Do. 



Do. 



Melting temperature, 1367 . 



From here on to the albite end of the series, viscosity becomes very 

 troublesome in restraining crystallization. The breaks which mark 

 the melting temperature on the heating curve of Ab 3 Ani are so slight 

 as to make the determination difficult and somewhat uncertain. It is 

 not that temperature measurement is less accurate here than else- 

 where, for these temperatures are more accessible than the melting 

 point of anorthite to which reference has been made in this connection. 

 These ultra-viscous materials do not melt at a constant temperature 

 but over a considerable range of temperature, as we shall undertake 

 to show in some detail, with illustrations from photographs, in the 

 discussion of albite. A glance at a series of curves (fig. 9) plotted 

 from our observations upon metallic silver and the feldspars An, 

 AbiAn 5 , AbiAn 2 and AbiAni, in such a way as to bring their melting 

 points together, will show clearly the nature of this difficulty. The 

 melting point of the metal is sharp, but with anorthite a change in the 



