Dr. Odliiig on, Orthu- and Meta-siitcates. 371 



In addition to these comparatively simple orthosilicates, which 

 can be readily formulated, there are a great number of complex 

 silicates, containing mixtures of several bases, proto-equivalent 

 and sesqui-equivalent, which evidently pertain to the formula 

 AP Si 0* or 2j\P . Si 0'^, because, despite great variations in 

 composition, the ratio of the oxygen in the base to the oxygen 

 in the anhydride invariably equals |. Such, for instance, are 

 harmatome, prehnite, chabasite, mesotype, &c. (Laurent). 



Class II. Metasilicates. 

 Type M^ Si 0^, corresponding to M P 0^ metaphosphate. 



K* Si 0^ Metasilicate of potassium (Yorke, Rose). 



Na*SiO^.H^O Metasilicate of sodium (Forehhammer). 



Ca« Si 03 WoUastonite. 



Mg* Si 0^ Picrosraine ; aphrodite. 



Ca Mg Si 0^ Augite. 



(CaMgFe)2SiO' Diallage. 



Fe^ Si 0^ Iron slag (Sefstrom) . 



Mn Fe Si 0^ Pyrosmalite. 



aP Si 03 . H2 Clay (Brougniart, Malaguti). 



fe'^ Si 03 Nontronite. 



Cu^SiO^.H^OChrysocolla. 



Class III. | Silicates. 

 Type W Si^ 0^ corresponding to M'* P^ 0'' pyrophosphates. 

 Na^ Si^ 0^ Silicate of sodium (Yorke). 



Mg^ Si^ 0" Ophite ; noble serpentine. 



af Si^ 0^ Clay (Forehhammer). 



(fl/MgFe)«Si2 0" Cordierite (Laurent). 



Ca'' aP Si^ 07 Sarcolite. 



Fe^Si^O^ Iron slag (Sefstrom). 



ft^ Si2 0^ H2 Hisingerite. 

 Fe^fl/^SisO^H^OOttrelite 



From the foregoing lists, we perceive that phosphates and sili- 

 cates are strictly comparable with one another. But a compa- 

 rison of the phosphoric acids with their corresponding silicic 

 acids is impossible, from our ignorance of the constitution and 

 properties of the latter acids, if indeed they have any existence. 

 But we should not, I think, expect to observe any decided differ- 

 ences in the three modifications, seeing that even in the phos- 

 phoric acids, despite the facilities afforded by their soluble con- 

 dition, the distinctions are not of a highly characteristic kind. 

 We might even speculate upon what our knowledge of the 

 varieties of phosphoric acid would now have been, if the ortho- 

 2 B3 



