398 J. D. Dana on the Feldspar group of minerals, 
6. That when there are several such derivatives, the Greek 
numeral prefixes di, tri, tetra, ete., may be used to indicate 
the number of molecules of water removed from the ortho acid 
New Haven, Oct. 10, 1867. 
' Arr. XLITI.—Crystallogenic and Crystallographic Contribu- 
tions; by James D. Dana. On the Feldspar group o 
inerals ; supplementary to Art, XXIX, page 252: On 
the Chemical Formulas of the Silicates. 
1. Chemical formulas of the Feldspars, and of some other Silicates. 
THE resemblance or affiliation between the species of. er 
Feldspar group is of the most intimate kind. There is 20 
Sees but also, apparently, the profoundest tsotypt 
oe Pome omnes 
ioxyds,” #8 
oxygen ratio between the protoxyds and sesquioxye* 
system, to the ratio haswoen thd paper a . “ayad 
Sesquioxyd or pseudotriad metals,” the so 
