  By means of high-resolution ultrasonic velocity measurements, as a function
of temperature and magnetic field, the nature of the different low temperatures
magnetic phase transitions observed for the quasi-one-dimensional compound
CsNiCl3 is established. Special attention has been devoted to the field-induced
120 degree phase transition above the multicritical point in the H-T phase
diagram where the elastic constant C44 reveals a step-like variation and
hysteresis effects. These results represent the first experimental evidence
that the 120 degree phase transition is weakly first order and contradict the
popular notion of new universality classes for chiral systems.
