48 W. E. Ford — Chemical, Optical and other Physical 



gravity yields what is termed the specific elasticity. In gen- 

 eral he finds that the value of the specific elasticity decreases 

 with increase of molecular weight. The column of the table 



F . 



headed ^ gives the values derived in this way for the garnets. 



In general they follow the rule given above. The variation of 

 the two is not as regular as might be desired. Their relations 

 are shown graphically in fig. 12. 



His second formula involves the use of the cubical content 

 of the optical indicatrix. In the case of the isometric minerals 



4 



this becomes --7m 3 , denoted by the symbol R. The quotient 



o 



-^ is known as the specific refractive power. In general it is 



found that this value is a constant for a given group of 



minerals. The column headed -^ gives these values for the 



garnets studied. Like the values derived from the Gladstone 

 Law they are reasonably constant until the andradite molecule 

 predominates, when there is a sharp break. The mean of 

 these values including all analyses up to number 21 is 5'993. 

 The average variation from this number is 0*135 or a dis- 

 crepancy of 2*2 per cent. 



Conclusions. — From this study of the minerals of the Gar- 

 net Group it is definitely established that the index of refrac- 

 tion and specific gravity of any garnet depend in a direct and 

 simple way upon its chemical composition. Further, having 

 given these physical constants of a garnet and knowing from 

 qualitative tests the predominant molecules present, it should 

 be possible in the majority of cases to predict within reason- 

 able limits the composition of the mineral. When the relation- 

 ships between specific gravity, refractive index and molecular 

 weight are studied they are seen to be fairly constant until the 

 andradite molecule becomes prominent. This causes a dis- 

 tinct break in the continuity of these relations. That the 

 relations between refractive index, specific gravity and mole- 

 cular weight of andradite is distinctly different from that 

 prevailing in the other garnets is unquestionable. Although 

 there are only a few analyses of andradite available with which 

 the refractive index is given, there are a considerable number 

 of analyses with which the specific gravity is given. To show 

 this variation in the case of andradite it is only necessary to 

 remember that andradite has the highest molecular weight of 

 all the garnets, and then to compare the specific gravities for 

 andradite as quoted by Dana in connection with the analyses on 



