120 Day and Allen — Isomorphism and Thermal 



men and therefore of the same chemical composition, was 

 imbedded beside the thermoelement as indicated in fig. 8. 

 This charge was heated slowly to exactly 1200°, slowly cooled 

 again and several thin sections prepared from the crystal frag- 

 ment and its immediate neighborhood. What the microscope 

 showed can best be seen from the accompanying illustrations 

 (Plate I, 2 et seq.), — groups of crystal fragments of micro- 

 scopic size, preserving their original orientation (extinction) 

 perfectly but with narrow lanes of glass where cleavage 

 and other cracks had been, forming a perfect network without 

 a trace of disarrangement. Considerable melting had taken 

 place but no flow. Neither had the charge as a whole made 

 any movement to take the form of the containing vessel after 

 sintering together (tig. 9). 



Surmising that we had accidentally hit upon the approxi- 

 mate melting temperature, a fresh charge of like material was 



Fig. 



Fig. 9. 



prepared, and the same experiment carefully repeated except 

 that the temperature was carried up to 1206° and maintained 

 there for 30 minutes. Instead of showing the melting to be 

 complete, the slides (Plate I, 3) looked precisely like the first 

 save that the lanes of glass were somewhat wider and the 

 crystal fragments relatively smaller than before. Further 

 trials under precisely the same conditions with the tempera- 

 ture increased to 1225° (Plate I, 4) and 1250° (Plate I, 5) 

 respectively, for like periods of time, showed only more 

 advanced stages in the same process. In the latter case the 



