204 On Heieronomic Isomorphism. 



by a cord passed once around it, the hand resting at the same time 

 on some firm object; whea all oscillation has ceased, it may be 

 let off by dropping the cord. In going the circuit, the ball of the 

 pendulum has always the same side towards the north, and con- 

 sequently it rotates on its axis with the earth. 



However careful the trial, the motion of the pendulum soon 

 appears to be somewhat elliptical. This has been attributed 

 solely to accidental causes, and it is true that the slightest error 

 of direction in letting off the pendulum will necessarily produce it. 

 But on a following page, Rev. C. S. Lyman shows that a degree 

 of ellipticity is a necessary result of the earth's rotation. 



J. D. D. 



L 



Art. XXIII. — Note on Heieronomic Isomorphism ; by 



Ja:mes D. Dana. 



In Volume Nine of thfs Journal* I published views on certain 

 isomorphous groups among minerals, tracing this quality to a re- 

 lation in atomic volume, a principle already admitted, but show- 

 ing that this relation is most correctly exhibited when the aggre- 

 gate atomic volume is divided by the number of atoms (or mole- 

 cules) of the elements present. Thus for Fe O, I would divide 

 the atomic volume, obtained in the usual way, by 2, which re- 

 duces the compound to the condition of a unit, as if consisting, 



as in effect it does, of Fe^ 0^. Again, for Fe^ 0^ I divided by 



3 3 /. 



5, as the compound consists essentially of Fe^ 0% the sum oi 

 the fractions making a unit. This meihod carried out with the 

 feldspars, exhibited a relation between them of actual (or approx- 

 imate) equality in atomic volume, and also a connection between 

 the system of crystallization of these species and the atomic 

 volume, — the monomelric (as Leucite) having the highest num- 

 ber, the monoclinic the next highest; and the triclinic a lower 

 number. 



Farther study is required before all the difficulties connected 

 with this subject, arising from isomerism and differences in the 

 elements, are mastered. But in the comparison of compounds 

 consisting of like elements though in different proportions, the 

 method appears to be satisfactory, and affords conclusions of great 

 simplicity. 



These views are well elucidated by many groups of silicates 

 and they give increased interest to the recent results of Ratn- 

 melsberg with the tourmalines. This distinguished chemist, 

 after numerous analyses, makes out jive chemical groups in 



* P. 220, I860. 



J 









