138 J. B. Ferguson and A. F. Buddington — 



index of refraction, positive optical character, and higher 

 birefringence with yellow interference colors. Similar 

 features have been observed and described by Smyth 9 

 in natural melilite crystals occurring in alnoite at Man- 

 heim, New York. 



Densities. — The densities at 25 °C. of both crystals and 

 glasses with the composition of akermanite, of gehlenite, 

 and of intermediate mixtures are given in Table III. 

 In fig. 3 they are plotted against composition in weight 

 percent. The densities were determined by the method 



Table III 



Densities of artificial crystals and glasses of the Akermanite- 

 Gehlenite series, at 25° C. 



Composition 



wt. percent. Density Density 



Akermanite Gehlenite. crystals. glasses. 



100 3.038 2.884 



25 75 3.018 2.903 



50 50 2.993 2.919 



75 25 2.969 2.938 



100 2.944 2.955 



of Day and Allen, 10 using the modified type of pycnometer 

 bottle suggested by Johnston and Adams. 11 The crystals 

 were prepared by heating a charge of appropriate com- 

 position over night a little below the temperature of be- 

 ginning of melting. The values are correct to ± 0.003. 



It will be noted that in the case of akermanite the 

 glass has a higher density than the crystals at a temper- 

 ature of 25 °C. This unusual relation is confirmed by 

 the relative values of the indices of refraction for glass 

 and crystal, the index for the former being a trifle higher 

 than the maximum index of refraction for the latter. 

 This does not necessarily mean that crystalline akerman- 

 ite contracts and becomes denser on melting, because the 

 coefficients of expansion of the glass and the crystal are 

 probably different and the difference between their densi- 

 ties at 25° C. is so small that their relative densities may 

 be reversed at the temperature of melting. 



Specific volumes. — In Table IV are given the computed 



9 C. H. Smyth, this Journal, 46, 104, 1893. 



10 Day and Allen, Carnegie Institution of Washington, Publication 31, 

 ' 1908. 



11 Johnston and Adams, Jour. Am. Chem. Soc, 34, 563-584, 1912. 



