140 Day and Allen — Isomorphism and Thermal 



of the two components must be a straight line. He also offers 

 a number of isomorphous pairs for which he finds the specific 

 volume curves to be straight lines, in support of his hypothesis 

 that this relation is general. Our values when plotted in this 

 way (fig. 22) also give a straight line with maximum variations 

 amounting to 0*005, which is probably not greater than the 

 aggregate error in the syntheses and in the determinations of 

 the specific gravity. 



In spite of this apparent corroboration, it does not seem to 

 us that Retgers was quite justified in assuming that this relation 

 is entirely without limitation. The temperature at which the 

 specific gravity is determined is so far below the temperature 

 of solidification (in our case, more than 1,000°) that the density 

 at 25° will depend to a considerable degree upon the coefficient 

 of expansion of the material as well as upon composition or 

 molecular structure. The coefficient of expansion will, in 

 general, differ for different substances, and is not, in general, 

 a linear function of the temperature. Considering Retgers' 

 generalization in the light of these facts, the relation of the 

 specific gravities at 25° would be necessarily continuous but 

 not necessarily linear. 



The specific gravities of the glasses are also plotted (fig. 21) 

 to show the divergence from the line of the crystals toward 

 the albite end of the series, i. e., as the percentage of albite 

 increases, the density of the glass is diminished more than that 

 of the crystals. 



There is nothing new in the conception of isomorphism in 

 the feldspars, but the positive character of our experimental 

 results makes them of more than ordinary interest by reason 

 of the fact that so good authorities on the subject as Fouque 

 and Levy have passed upon it adversely on the basis of optical 

 evidence derived from artificial preparations. More recently 

 Viola,* has declared that the optical evidence is insufficient 

 to prove isomorphism in the natural feldspars. 



The melting points and specific gravities are brought together 

 in a convenient table here. 



Feldspar An 



Melting temp. 1532° 

 Sp.gr. (crystals) 2*765 

 Sp. gr. (glass) 2 -700 



(3) In the melting of albite and microcline we appear to have 

 substantial evidence of a phenomenon which is unfamiliar both 

 to physics and to mineralogy. Microscopic crystals of a homo- 

 geneous compound, when slowly heated, were shown to persist for 

 150° or more above where melting began, the amorphous melt 

 remaining of the same order of viscosity as the rigidity of the crys- 



* Loc. cit. 



AbiAn 5 



Ab a An 2 



AbxAii! 



Ab 2 Aiii 



Ab 3 An! 



Ab 



1500° 



1463° 



1419° 



1367° 



1340° 





2-733 



2-710 



2-679 



2-660 



2-649 



2*605 



2-648 



2-591 



2-533 



2-483 



2-458 



2-382 



