52 ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS 



strain was greatest. On the other hand, a variable extinction angle 

 in an unbroken crystal fragment frequently gave unmistakable evi- 

 dence of the bending of the crystal as well as the vitreous portion. 

 From these qualitative experiments it seems possible to assert with 

 confidence that the order of magnitude of the viscosity of the molten 

 portion (glass) is the same as that of the rigidity of the crystals at 

 these temperatures. Plate XXIV shows a piece of Mitchell County 

 albite heated to 1200 under load. The sagging is indicated by the 

 curved cleavage cracks. A sliver of microcline, similarly treated, is 

 reproduced in Plate XXV. The displacement is shown by the curva- 

 ture of the crystal edges and the cleavage cracks ; the black portions 

 are glass. It is interesting to observe that while the crystal has 

 melted completely across, there has been no displacement of the 

 cleavage plane (indicated by a clotted line). 



Plate XV is from a charge of composition Ab 3 Ani which had been 

 heated to 1375 and completely melted. It was then allowed to cool 

 slowly in the furnace. On the following clay it was reheated to about 

 1250 for most of the day. The slide was made from this mass. The 

 dark portions of the slide are glass in which the crystals were induced 

 by the subsequent reheating. At first sight it would seem that crys- 

 tallization ought to be complete after the mass had been allowed to 

 cool in the furnace and had been reheated for six hours at a tempera- 

 ture within 125 degrees of its melting point, but the slide plainly 

 shows that equilibrium is reached very slowly in melts of this extreme 

 viscosity, even after nuclei have formed. 



The preceding experiments gave a clear idea of the phenomena 

 attending the melting of albite and orthoclase, and convinced us that 

 the absorption of heat accompanying fusion, which we had searched 

 for in vain upon the heating curves in the earlier experiments, had 

 eluded us merely because it was extended over so long a stretch of the 

 curve as not to be noticeable. Some very exact measurements of the 

 temperature change from minute to minute were therefore made in 

 the hope that a more intelligent search might be more successful. 

 Separate charges of glass and of crystals of the same composition and 

 of equal weight were prepared and successively heated in the same 

 furnace with the same current. The specific heat is, of course, not 

 identical in the two cases, but the curves were comparable in form. 

 Above 1100 we felt sure that one of the curves must contain an 

 absorption of heat which would be absent from the other. Such a 

 pair of curves (I), taken from the microcline measurements, is repro- 

 duced in the adjoining figure (fig. 12), and appears to show such an 

 absorption clearly, extending from 1135 to 1275 . The dotted line 



