W. E. Ford — Chemical, Optical and other Physical, etc. 33 



Aet. III. — A Study of the Relations existing between the 

 Chemical, Optical and other Physical Properties of the 

 Members of the Garnet Group • by W. E. Ford. 



It is well recognized that one of the important mineralogical 

 problems at present demanding solution is the establishment 

 of the relations that exist between the chemical composition 

 of a mineral and its physical properties. No one doubts but 

 that a definite relationship does exist between them since 

 that fact has been abundantly proved in various isolated cases. 

 The complexities of the general problem are, however, so great 

 and the exact data at hand so limited that as yet it has been 

 impossible to give any satisfactory statement of the relations 

 involved. One attempt has been made in the so-called Glad- 

 stone Law to correlate the chemical composition of a mineral 

 with its index of refraction, specific gravity and molecular 

 weight. In certain cases the assumptions of this law gave 

 reasonably close results, but in others the discrepancies were 

 very great. The Gladstone Law ignored any possible influ- 

 ence that the physical structure of the molecules and the 

 crystal net-work in which they are grouped might have upon 

 the physical properties. That such molecular characters do 

 influence the physical properties of a mineral is proven by 

 their variation in the different cases of polymorphism. It is 

 very probable that any law that would correlate the chemical 

 and physical properties of the minerals belonging to one crys- 

 tal system would have to be modified in order to apply it to 

 another system. Further, it would seem also probable that not 

 only the arrangement of the crystal particles in a particular 

 net-work but also the structure of the individual molecule or 

 group of molecules that forms the unit of the net-work would 

 have a definite effect upon the physical properties. In other 

 words, it seems reasonable to suppose that while the chemical 

 composition of a mineral has undoubtedly a predominating 

 influence, its effect is modified by these other factors. Very 

 possibly any general law that may be derived will have to be 

 modified by the inclusion in it of factors that would vary not 

 only for the different crystal systems but also for the different 

 minerals or groups of minerals. 



At the present time such considerations as the above are 

 almost purely speculative in nature. Before anything like a 

 satisfactory general statement can be made, a far greater 

 amount of accurate data than at present available must be 

 accumulated as well as the further study and correlation of 

 the facts already known. The present investigation is simply 

 a contribution to the preliminary study of the general problem. 



Am. Jour. Sci. — Fourth Series, Vol. XL, No. 235.— July, 1915. 

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