  In glass, starting from a dependence of the Angell's fragility on the Poisson
ratio [V. N. Novikov and A. P. Sokolov, Nature 431, 961 (2004)], and a
dependence of the Poisson ratio on the atomic packing density [G. N. Greaves et
al., Nat. Mater. 10, 823 (2011)], we propose that the heterogeneities are
predominantly density fluctuations in strong glasses (lower Poisson ratio) and
shear elasticity fluctuations in fragile glasses (higher Poisson ratio).
Because the excess of low-frequency vibration modes in comparison with the
Debye regime (boson peak) is strongly connected to these fluctuations, we
propose that they are breathing-like (with change of volume) in strong glasses
and shear-like (without change of volume) in fragile glasses. As a
verification, it is confirmed that the excess modes in the strong silica glass
are predominantly breathing-like. Moreover, it is shown that the excess
breathing-like modes in a strong polymeric glass are replaced by shear-like
modes under hydrostatic pressure as the glass becomes more compact.
