Properties of the Garnet Group. 47 



In the first column of the table (p. 40) are given the indices of 

 refraction of the different garnets arranged in an ascending 

 series. The second column gives the corresponding specific 

 gravities. It is seen that in a general way the specific gravity 

 increases with rise of the refractive index, although this cor- 

 respondence between the two is by no means constant. There 

 is, moreover, a sharp break in the continuity beginning at 

 analysis 21. It is evident from this that garnets which con- 

 tain large amounts of the andradite molecule show decidedly 

 different relations between these two properties than the other 

 garnets. This is readily seen when the fact is noted that 

 andradite with the highest refractive index of all the garnets 

 has a specific gravity lower than either spessartite or almandite. 

 The relations between the refractive indices and specific gravi- 

 ties for the series is shown in fig. 9. 



In the third column of the table are given the figures 

 obtained by use of the constant K of the Gladstone Law, which 



n — 1 

 equals — ~ — • The results here were reasonably concordant 



until the garnets containing the andradite molecule were 

 reached. These naturally showed a distinct variation. Taking 

 the average of the numbers obtained from the analyses through 

 almandite we obtain as a mean value, K= *1986. The average 

 variation from this mean is '0064 or one of 3'2 per cent. 

 From the above it is seen that with the exception of andradite 

 the members of the garnet group show reasonably constant 

 relations between specific gravity and refractive index. 



The fourth column of the table gives the molecular weights 

 of the various garnets as determined from the percentages of 

 each molecule present in then). In general it is seen that the 

 indices of refraction and the molecular weights of the series 

 through almandite increase together. It is also evident that 

 the molecular weights and specific gravities have similar rela- 

 tions. From figures 10 and 11, it is seen, however, that these 

 relationships are not very exact. 



Eosicky* has recently briefly discussed the relations between 

 refractive index and specific gravity in minerals and has sug- 

 gested new formulas for use. He makes use of what he terms 

 (1) the measure of the optical elasticity and (2) the measure of 

 the strength of the refraction. For the measure of the optical 

 elasticity he derives the cubical content of the Fresnel ellip- 

 soid. In the case of an isometric mineral this becomes a 



4:7T 



sphere and the constant, designated as F, becomes -^-^ (n = 



refractive index). This expression divided by the specific 

 *Bull. Acad. Sc. Boheme, 1911, 



