20 



absorbed nothing, would therefore be 2-232. According to Smitlison, 

 the specific gravity of zinc bloom, is 3 '59. 



CHEMICAL COMPOSITION OF THE SILICATES OE ZINC. 



Pisolitllic Amorphous Silicates. — "We shall first speak of the piso- 

 lithic silicates, the formation of which is described at page 10. Some 

 of these balls are opaque, and consist of beautifully concentric shells ; 

 but nearly all that we have examined contained a semi-translucent 

 opal-like nucleus, often not bigger than a pin-head, but sometimes as 

 large as the largest-sized peas ; sometimes spheroidal balls, as large as 

 beans, of this opalescent silicate, are found. These opalescent nuclei 

 and balls are not, like the opaque ones, composed of concentric layers, 

 but appear to be quite homogenous. The concentrical structure, as well 

 as the opacity, may, perhaps, in some cases be explained as a process 

 of drying, or dehydratation, and not as a successive growth ; in favour 

 of this view is the fact, that the opalescent nucleus has generally 

 somewhat more water than the opaque external shell. In some cases 

 this explanation does not certainly apply ; for the nucleus has a different 

 composition from the opaque shells, and the latter have all the appear- 

 ance of having been successively formed about the former — ^the external 

 surfaces of some of the shells having different lustres, for instance. The 

 following are the results of the analyses of several of these balls : — 



I. — Slightly spheroidal ball, not found as a nucleus, but may have 

 been originally in a large ball ; lustre resinous, inclining to vitreous ; 

 fracture conchoidal and shining ; colour, milk-white ; semi-translucent ; 

 brittle ; sp. gravity, 3*694 ; not unlike opal, but not iridescent. 



II. — A remarkably round ball, 6 to 7"*™ in diameter, pure enamel- 

 white ; surface smooth, exactly like glazed porcelain, or fused white 

 enamel ; fracture like biscuit porcelain. 



III. — Ball of about the same size as 'Eo. II., but having a dull sur- 

 face ; colour, enamel- white ; fracture like biscuit porcelain. 



IV. — A pea, 5"™ in diameter, taken from the centre of a large ball 

 IQmm -j^ diameter ; external surface smooth, like fused enamel ; fracture 

 like biscuit porcelain ; colour, pure .enamel- white ; streak, white ; hard- 

 ness, 3*5 ; sp. gravity, 2*883. It contained in the centre a semi-trans- 

 lucent nucleus, about the size of a mustard- seed, of the density and other 

 properties of ]^o. III. 



I. 



Oxide of zinc, .... 64*549 



Silicic acid, 6-493 



Carbonic acid, .... 11-246 

 Iron in combination 

 with phosphoric acid, 



Lime, 0-006 



Magnesia,"! 

 Alkalies, / 

 Water, 17-672 



0-003 



traces 



99.969 



IL 



61-865 

 8-292 

 11-301 



0-002 



traces 

 traces 

 18-624 



100-084 



III. 

 62-266 

 9-214 

 10-101 



0-003 



0-001 



traces 

 19-362 



100-947 



IV. 



66-844 

 17-471 

 4-637 



0-002 



traces, 

 traces. 

 10-834 



99-788 



