74 



THOMAS STEElîY HUNT ON A NATURAL SYSTEM IN 



Church, in 1875, coufii-med the previous statements as to the differences in density among 

 the minerals included iinder the name of zircon. Thus he found the hyacinth-red crystals 

 of Expailly to have a specific gravity, 4.863, which was not changed by ignition. A large 

 niimber of zircons examined by him. not less than twelve in number, varied from 4.60 

 to 4.70, while an opacjue brown zircon fiom North Carolina had a density of 4.54, which 

 was changed to 4.67 by long ignition, and a transparent brown zircon from Frederickviirn 

 had its density by the same process raised from 4.48 to 4.63. Another zircon, dark green 

 in color, slightly opalescent and flawless, had a specific gravity of only 4.02, w^hich was 

 not changed by ignition. Jt was, ncA-ertheless, according to Church, a true zircon, giving 

 by analysis the percentages of that species.' Auerbachite, an isomorjihous zirconic species, 

 has, with different atomic ratios, a specific gravity of 4.05, and agrees in volume with 

 malacone, a hydrous zircon, which has a similar density, while other related hydrous 

 zirconic silicates give specific gravities of from 4.00 to 3.60. It would appear that the 

 zirconic, like the alr;minic adamantoids, exhibit species A'arying alike in atomic ratio, in 

 condensation, and in the presence and absence of water. An anhydrous zircon, with the 

 ratio of 1 : 1, and a density of 4.86, has an atomic volume of 4.68 ; and one of 4.70, a volume 

 of 4.84 ; while a zircon, with the lower density of 4.02, has a volume of 5.65. This last we 

 may distinguish by the trivial name of " lyucurite," from the lyncurion of Theophrastus,- 

 while the denser zircon of Expailly may also, jjerhaps. recxuire a distinctive name. 



Tribe 14. — Perphylloid. 



§ 97. As regards the phylloid tribe of the persilicates, an important chapter of their 

 history is connected with pholerite and kaolinite. It was in 1825, that Guillemin 

 described under the name of pholerite a hydrous silicate of alumina, micaceous in struc- 

 ture, to which he assigned the atomic ratio for alumina, silica and water of 3:3:2. 

 This was the same as that deduced from the analyses of Brongniart and Malaguti for 



XIII. 



Tribe 14. Perphylloid. 



ordinary kaolin, although Forchammer had proposed for the latter the ratios, 3:4:2. 

 The uncertainty as to the composition of these silicates which prevailed thirty years since, 

 is reflected in the fourth edition of Dana's " System of Mineralogy," published in 1854, 

 where the first-named ratio was ascribed to kaolin, with the remark that it occasionally 

 presents the second ratio ; while of pholerite it was said " that it does not differ much from 



'Cliiinli on Densities of Precious Stones; Geolniriral jSIag.izino for I.'!"-" 

 - Moore's Ancient Mino.ralofry, p. 14.-). 



