100 



'ay am 



ten — Isomorphism am 



lermat 



Later on, when more experience had been acquired, these 

 minerals were taken up again and a satisfactory explanation for 

 their behavior was found. But for the moment all the defin- 

 ing phenomena appeared to be so effectively veiled by some 

 property, presumably the viscosity, that we were constrained 

 to look about for some simpler compound which should give 

 us a better insight into the behavior of mineral glasses and 

 their thermal relations, and to lay aside the feldspars until 

 they could be more successfully handled. 



This outline of our unsuccessful experiences is given here in 

 some detail, in order to show the actual difficulties which con- 

 front the student in working with the feldspars, in the face of 

 which it is certainly not surprising that uncertain and contra- 

 dictory conclusions have been reached. 



Borax. — The substance chosen for this preliminary work 

 was ordinary anhydrous borax (sodium tetraborate). We chose 

 this merely because it was a simple glass and unlikely to 

 undergo chemical change. It is easily obtainable pure and its 

 thermal phenomena are within easy reach. The study of 

 borax proved to be most instructive. It gave us an effective 

 insight into the behavior of this class of substances, and in 

 particular served to define the phenomena of melting and 



solidifying in substances which 

 undergo extreme undercooling and 

 which recrystallize with difficulty or 

 not at all. The results of this study 

 of borax were therefore of much 

 interest in themselves and were given 

 in a paper before the National Aca- 

 demy of Sciences at its spring meet- 

 ing in Washington last year (April 

 21, 1903), but were not printed at 

 that time. 



The borax glass upon which our 

 measurements were made was pre- 

 pared in the Usual way by heating 

 the crystals until the water of crys- 

 tallization had been driven off and 

 the viscous mass was reasonably free 

 from bubbles. If the borax is pure, 

 Fig. 1. the anhydrous product, when cooled, 



is a brilliant, colorless glass, iso- 

 tropic, of conchoidal fracture, and specific gravity 2*37. The 

 specific gravity was determined in the fraction of kerosene 

 boiling above 185° C. About 100 gr. of this glass were 

 then broken up and placed in a platinum crucible in the 

 electric furnace. The thermoelement was placed in position 



