  The magnetic field dependence of the low frequency dielectric constant
$e_r$(H) of a structural glass a - SiO2 + xCyHz was studied from 400 mK to 50
mK and for H up to 3T. Measurement of both the real and the imaginary parts of
$e_r$ is used to eliminate the difficult question of keeping constant the
temperature of the sample while increasing H: a non-zero $e_r$(H) dependence is
reported in the same range as that one very recently reported on multicomponent
glasses. In addition to the recently proposed explanation based on
interactions, the reported $e_r$(H) is interpreted quantitatively as a
consequence of the disorder lying within the nanometric barriers of the
elementary tunneling systems of the glass.
