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LXXI. On the Form o/' Eudialy te. Bj/ Professor Miller*. 



THE angle between normals to two adjacent faces p, of one 

 of the rhombohedrons of eudialyte, is stated to be 106° 20' 

 in some mineralogical treatises, and 106° 36' in others. A 

 very accurate determination of its form is difficult on account 

 of the unevenness and dullness of its faces. In order if pos- 

 sible to render it a little less uncertain, I measured some very 

 perfect crystals, for which I am indebted to Dr. A. Smith of 

 Dublin, and also some of the best crystals in Mr. Brooke's 

 collection. The resultinf; values of the an^le between nor- 

 mals to the faces o and p, either given immediately by ob- 

 servation or computed from the angles between other pairs of 

 faces, after excluding all the observations that were unsatis- 

 factory on account of the largeness of their difference from 

 the mean, or the indistinctness of the reflected images, are 

 67° 42'— 40'— 44'— 48'— 41'— 39'— 4-5'— 41'— 35' — 43'— 41' 

 —47'— 41'— 36'— 40' — 47'— 42'— 47'. The angles between 

 normals to the faces calculated from the mean of the pre- 

 ceding values of op, are 



u 



90° 0' 



c u 



30° 0' 



DC 



90 



zz' 



5S 35 



Z 



31 22 



X x^ 



84 4 



OX 



50 38 



pp' 



106 30 



op 



67 42 



ss' 



116 4. 



o s 



78 25 



71 t 



13 59 



of 



81 11 



pt 



22 46 



The symbols of the forms to which the different faces be- 

 long are, in the notation which I have adopted in my treatise 

 on crystallography, 



o {111}, c {211}, « {lof}, p {100}, a; {no}, 



z {211}, s {111}, t {201}. 



The faces t, which occur on two of the crystals in the pos~ 

 session of Mr. Brooke, do not appear to have been observed 

 beiore. 



St. John's College, Cambridge, 

 May 6, 1840. 



* Coram imicated by the Author. 



