  We give a simple demonstration of the formula relating the glass transition
temperature, $T_g$, to the molar concentration $x$ of a modifier in two types
of glasses: binary glasses, whose composition can be denoted by
$X_nY_m+xM_pY_q$, with ^$X$ an element of III-rd or IV-th group (e.g. B, or Si,
Ge), while $M_pY_q$ is an alkali oxide or chalcogenide; next, the network
glasses of the type $A_xB_{1-x}$, e.g. $Ge_xSe_{1-x}$, $Si_xTe_{1-x}$, etc.
After comparison, this formula gives an exact expression of the parameter
$\beta$ of the modified Gibbs-Di Marzio equation.
