Expansion effects shown by some pyrex glasses

Thermal expansion data are presen ted , which apparently confirm a previously expressed belief t hat some of t he borosilicate glasses containing a high percent age of silica are essent ially two-component glasses. One of t hese compon ents is presumably composed almost ent irely of silica and forms into a comparatively rigid and con tinuo us spatial network t hat pervades t he whole gla ss. This forma tion develops before t he glass is cooled into t he anneali ng ran ge of t h e other co mponent t hat is composed o f a por tion of the silica ann most of t he other constituents. On bein g cooled into i ts annealing range and parti cularl y on reaching te mperat ures below this range, t he low-temperatu re componenL also forms a fairl y rigid network that is interlaced wHh t ha t of t h e l{igh-temperature component. Th e lat ter , bei ng co mposed mainly of silica, possesses a considerabl y lowe r expan sivity t han t he other. Consequen tly, the low-temperat ure componen t , whenever it beha ves as a rigid network, elastically compresses t he other component on cooling and is, in t urn , elastically distended . Annealing t he glass a t s ui table te mperat ures relaxes t he compressio ns and distensio ns. Such r elaxations cause expan sions and co n tract ions t hat are unlike those fou nd in a normal single-compon ent glass. As a res ult, thermal expansion cycles obtained on glasses containing a high per cent a ge of sili ca show certain peculiarities. Several cases in which these peculia rities appear are presen ted a nd discussed.