68 



ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS. 







100 A 



Composition 

 Fig. 22. 



100 B 



lease of latent heat does not raise the temperature above c again). 

 Such a situation is certainly unavoidable in viscous mixtures like the 

 feldspars and accounts very well for the homogeneous solidification ob- 

 served by us. This would classify the feldspars with Type I of Rooze- 



boom's series. A comparison of our 

 melting-point curve with figs. 19, 

 20, and 2 1 shows this to be the only 

 type under which it could possibly 

 fall. There is no trace of a maxi- 

 mum or minimum in the feldspar 

 curve. Vogt's expectation that they 

 would fall under Type III, therefore, 

 fails of fulfilment from our experi- 

 ments. 



That our curve so closely resem- 

 bles one branch of Roozeboom's 

 typical curve is remarkable. The 

 difficulties of observation in those 

 portions of the curve where the 

 viscosity becomes so disturbing are 

 too great to enable stress to be laid 

 upon the form which our curve happens to take there, but near the 

 anorthite end of the series its slight convexity is unquestionably real. 

 It should be added that Professor Iddings has found slight traces of 

 inhomogeneity (less than 1 per cent) in the slides of several of our 

 intermediate feldspars. Crystals have been found which were evi- 

 dently of the earliest formation, and with one exception were more 

 calcic than the body of the charge, as Roozeboom's theory would lead 

 us to expect. The exception was an occurrence of tiny plates of 

 Ab 4 Aiii discovered in a charge of Abi An 5 . The extremely small quan- 

 tity of the optically different feldspar, the fact that it could not be 

 found in all the slides of this composition, and that in one case a less 

 calcic feldspar appeared, suggest that the inhomogeneity may have 

 been of other origin perhaps merely a consequence of the tremendous 

 difficulty in mixing a homogeneous charge where ultraviscosity pre- 

 cludes stirring, for example. The chemical analysis of the solid and 

 liquid phases, it will be remembered, showed identical composition 

 within the limits of experimental error. 



It is clear that if Roozeboom's theory is valid, the line of the melt- 

 ing points can not become perfectly straight unless the --curves for 

 the solid and the liquid phases can be superposed point for point 

 throughout, i. e., are identical. This would mean that the energy 



