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A m odel was co nstru cted of p entagon al rin gs o f sy mm et ric al tet ra hedra so co nn ected t ha t all fiy e silico ns li e in one plane and all angles Si-O-Si app rox imate 180°. From an ini tial tet rahedro n t he rings extend in six direction s to include its six edges and in t he r es ul t in g t hree-dim ensional network ea ch rin g forms a commo n in te rface b e t,~ee n d od eca · hedral cages w hi ch necessar il}: ha ve fiv efold symmetry , Such sy mmetry is in co mpat ible WI th formatJOll of a cryst al lattIC e and t hu s t he stru cture can be exte nded in t hree dimensio ns only to a very limi ted deg ree and wi th unavoidable stress and distor tional s t rain , A stressed net work clu ste r of s uch dodecahedral cages is here called a Vl'I' R ON , If t he scale in t his m od el for silica gla ss is t aken a s 1.6 A, fr om s ilico n t o oxyge n t he model is found to be in acco rd wi t h othe r radia l di stan ces compu ted fro m diffractio n 'data a nd t he d ens ity is correct wi t hin 10 p ercent . The apertures o f t he p entagonal in terfaces a re of s ui table size to explain data on the diff usion of noble ga ses t hrough sili ca , Sha red pent a fa ces a nd in termi ttent oxyge n b ridges connect neig hb oring vi t ron s a nd co ns t it ute a n in t erst itial t iss ue of r ela tively wea kened st ru ctu re t ha t a cco un ts for t he obser ved low te nsil e st rengths of s ili ca glass a nd p rov ides cha nn els fo r viscous fio\\', Th e prop osed cages Il'o ul d be less plia bl e at low te mperat llres a nd so t he in terst it ia l t iss ues betwee n " it rons s houl d expa nd in I'olume on coo li ng, This " negative" ex pa nsion wo uld cO ll nte r t he norma l ex pa nsio n a nd acco un t fo r t he know n ext remely s ma ll (ne t) e x p a n s i vit~' o f sili ca glass , The acco mpan y in g chan ges i n st re ngth a nd volum e of t he gla ss at its \I'eak es t places, t he i nte rst it ial t iss ues, ca n acco un t for a number of kn own " a nomalies" i n t herma l be havior of s ilica glass \I' it h re,pecL to i ts volume, co mpressibi lity, clast ic mod uli , a nd l'isco us fl ol\'. . The v itron co ncep L r eco ncil es t he crystaJli te an d t he network theories of glass by p rop o s in~ llu clei t ha t cannot g rOll' extell sil'ely and a dist ri b u t io n of locali zed st resses t hat m a y co nstit u te a n a ccep tabl e degree o f randomn ess a nd prov ide effective modul ations in co nt inui ty, This co ncept o f d e fi nite bu t limi ted microreg ulal'ity i n stru cture s u g~ests t hat other noncr,vs tal s Y lllmet ri e~ sho uld be s tud ied for p oss ibl e inLe res L in ot her fi eld s s uc h a s li q u ids a nd hi g h p ol,v mers,

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Air as a standard reference medium for precise refractive-index measurements is discussed with respect to the precision necessary in the control and measurement of its temperature, pressure, humidity, and carbon-dioxide content. Particular attention is given to the selection of provisional values for <>, the optical temperature coefficient for air; and the preparation of accurate correction tables for the reduction of refractive-index observations to standard conditions of reference is described and exemplified.The approximate range of temperature effects on absolute refractive index is indicated; also the range of pressure effects on liquids. A new (empirical) relation between index and density is derived from Pockels' optical data on elastically deformed glass and a basis for quantitative treatment of permissible stress-birefringence is given. For glasses, the character of annealing and the permissible degree of striation are considered; and for several media the requisite constancy in wave length of light source is determined.Tolerance equations are given for all requirements that are quantitatively discussed and it is concluded that both precision and accuracy to within ± 2 or 3 X 10-6 can be attained in determining refracti ve index by the classical method of minimum deviation. CONTENTSPage d == densi ty ;1 Brief historical references have been made in a ormer paper, BS I,Research 2, 912-915 (1929) RP64 ,