4 8 



ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS. 



The specific gravity determination gave 2.615, which, when cor- 

 rected for the small quantity of sodium tungstate becomes 2.612. If, 

 as is possible after the above treatment, the tungstic acid is present 

 as the anhydride, sp. gr. 7.1, the correction would lower the value 

 to 2.605, m excellent agreement with the other determinations. 



The products of both methods of purification were carefully scrutin- 

 ized by the microscope, but no conclusion could be reached as 

 to which was the purer. Neither the sodium sulphate fusion, nor 

 the digestion with acid and ammonia appeared to have changed the 

 particles in the slightest degree. Diligent search was made for opaque 

 or amorphous matter on the surface of the grains, or any other indica- 

 tion of decomposition, but none was found. While the chemical anal- 

 ysis indicates a rather higher purity for the first product, purified by 

 fusion, the differences are nearly within the limits of error and, there- 

 fore, hardly conclusive. Both powders were ground finer than usual 

 for the specific gravity determinations to avoid errors introduced by a 

 spongy structure. 



Reverting now to Hautefeuille's directions, it is clear that glass of 

 albite composition crystallizes homogeneously under substantially the 

 conditions which he obtained, as well or better than the mechanically 

 mixed component parts ; but the part played by the tungstate requires 

 some further experimental study before a conclusion can be reached . 



Except for the specific gravity, the experiments upon crystalline 

 albite and orthoclase which follow were made upon natural specimens 

 from well-known localities (a fragment of the Mitchell County albite is 

 shown in plate XVIII), for which we are indebted to Dr. G. P. Merrill of 

 the United States National Museum and Dr. Joseph Hyde Pratt, State 

 Mineralogist of North Carolina. The specimens were selected with 

 great care, but like all natural specimens, they contained other feld- 

 spars and inclusions. The analyses follow : 



