Appli cations of v it ron t heo ry to analyses of properti es of R 20 s ili cate binary glasses indicate that change in slop e on t he property-composition curves m ay occ ur where t h ere are integral numbers of cation s p er dodecah edral cavity or cage of th e network. From geometrical consid eration s i t is found that 1 or 2 oxygens and 6 sodiums can be accommodated inside a cage, an d similar ly not more t h an 1 oxyge n a nd 4 potassiums can be inclosed in a cage.Data on ch emi cal attack and on solubility losses in water s ugges t for alkali s ilicate glasses a maximal a verage inclos u re of oxygen inside t he cavit ies at 16.7-mole p ercen t for K 20 , 23. 1 percen t for Na20, and probably 28.6 p ercent for Li 20 . T hese composit ions correspon d to 1, 1.5, an d 2 oxyge ns per cage. T hese co mposit ions m ark t he beginning deterioration of the net. poss ibl y because of non bridg in g oxygen. C hanges in rate of volatili zation occ ur at 28.6-, 37.5-, a nd 50 .0-per cent R ,O for potash , soda, and li t hi a sili cate glasses, res pectively. These crit ical co mposit ions are cons id ered t hose for which t he cavit ies in t he p;lasses are com p letely saturated wit h the modifi er oxides, R,O, and with additional cations, R .D etail ed analyses of cur ves of molar volume versus co mposition show th a t the critical points for oxide saturatio n arc evide nt and that ch a nges in slope may occur at other cri t ical compos ition s corresponding to in tegral numbers of cat ions per cage. It is found that t h e percentage of 16.7-percent N a 20 at hi gh temperatures and the sat ura t ion point of 37.5 perce n t at low temperatures are cri t ical in curves for specific (electri cal) resistance. The "deep-well " point at 16.7-perce n t Na20 critically separat es soda sili cate glasses t hat are very rea dily devitri fi ecl from those t hat contain more oxide modifi er a nd are more stable.