80 THOMAS STEKRY HUNT ON A NATUEAL SYSTEM IN 



volume, if correctly determined, would be the same. He, however, pointed out that in 

 these oxyds, as iu organic compounds, examples of polymerism might be looked for, and 

 cited as examples cuprous and arsenious oxyds which (like ziucite and tridymite) haA'e 

 A'olumes which are much greater. Gerhardt, however, left untouched the question of the 

 molecular weights of these more or less condensed oxyds thus compared by him. 



§ lOG. Near in condensation to the great group of oxyds thus studied by Gerhardt, is 

 the rhombohedral species, quartz, which with D, 2'6ô, gives V, -S'GB, while the tetragonal 

 cassiterite, D, d'Yô, gives V, 5*55. Coming next to titanic oxyd. we find for the tetragonal 

 rutile, and the orthorhombic brookite alike, with D, 4'20, a value lor V of 4'88 ; while the 

 tetragonal octohedrite, if we take D, 3"82, (which is also near that assigned by several 

 observers to arkansite) gives for V, .536. A still more condensed molecule than that of 

 rutile appears iu the rhombohedral corundum, which with D, 400, gives V, 4'25, while 

 the hydrous adamantoid diaspore, orthorhombic in form, if we take its maximum observed 

 .specific gravity, 350, gives for V, 428. Near to these is the orthorhombic protoperoxyd, 

 chrysoberyl, which, with D, 3'80, gives Y, 418. Thus rutile and brookite are near in 

 atomic volume to spodumeue, sapphirine, staurolite, andalusite, librolite, bucholzite and 

 zircon, (V = 4.80-490), while cassiterite and quartz are near to the spinel group, and to 

 chrysolite, pyroxene, garnet, epidote, beryl and lyncurite. Corundum, diaspore and chry- 

 soberyl stand apart from all of the.se as having a more condensed molecule than even 

 cyanite and xenolite, the most highly condensed silicates known. 



§ 107. While small differences in atomic volume may, as Grerhardt insisted, be set 

 down to impurities and errors in determination, a careful survey of many silicates and 

 oxyds leads to the conclusion that among these there are great groups which, essentially 

 agreeing among themselves in molecular condensation, differ in the value of V from other 

 groups by quantities less than those admitted as accidental variations iu volume in the 

 large series brought together Ijy Gerhardt ; which may thus very \vell be found to include 

 two or more distinct groups with unlike volumes. At the same time, the comparisons 

 which we have here made among the adamantoid oxyds, not less than those among the 

 various tribes of silicates, serve to strengthen the conviction that the accident of 

 geometric form, however valuable as a means of diagnosis, is of altogether minor import- 

 ance iu investigating the general relations' of mineral species. 



^^ 108. The metals proper, together with the bodies of the sulphur and the arsenic 

 series, and the various binary and ternary compound of all these, make up the great natural 

 order of Metallates, which inclvides two suborders. Of these the first or Melallometa/lates, 

 distinguished by opacity and metallic lustre, is divided into six tribes, which are : — 

 1. Metalloids. — native metals and metal-like elements ; 2. Galenoids, — argentite, galenite, 

 boruite, chalcocite, metacinnabar, onofrite, stibnite, etc. ; 3. Pyritoids, — ^pyrite, linua-ite, 

 chalcopyrite, pyrrhotite, etc. ; 4. Smaltoids, — smaltite, niccolite, breithauptite, with other 

 arsenids, antimonids, etc. ; 5. Arsenopyritoids, — including arsenopyrite, cobaltite, etc. ; 

 6. Bournonoids,— enargite, bournonite, zinkenite, etc. The various selenids and tellurids 

 form subtribes distinct from the sulphurous or Thiogalenoids. In the second suborder 

 are included those species more or less resinous or adamantine in lustre, generally red in 

 color or in streak, and often transparent or translucent, whence the distinctive name 

 of SpathomeUdlates. In this suborder we distinguish at lea.st two tribes : 1. Sphaleroids, 

 — corresponding to galenoids, and including cinnabar, realgar, christophite. marmatite, 



