  A solid - amorphous or crystalline - is defined by a finite shear modulus
while a fluid lacks such. We thus experimentally investigate the elastic
properties of a colloidal glass former near the glass transition: spectroscopy
of vibrational excitations yields the dispersion relations of longitudinal and
transverse phonons in the glassy state. From the long wavelength limit of the
dispersion relation we extract the bulk and the shear modulus. As expected, the
latter disappear in a fluid and we measure a clearly resolved discontinuous
behaviour of the elastic moduli at the glass transition. This not only
determines the transition temperature T_G of the system but also directly
addresses recent discussions about elasticity during vitrification. We show
that low frequency excitations in our system are plane waves such that
continuum elasticity theory can be used to describe the macroscopic behaviour.
