  We present experiments showing that a granular system of small beads does
exhibit ageing properties : its maximum stability angle is measured to increase
logarithmically with resting time, i.e. the time elapsed before performing the
measure. We show that humidity is the crucial ingredient responsible for this
behaviour : while ageing effects are important at intermediate humidity, they
disappear at vanishing humidity. On the basis of these experimental results, we
propose a model based on the activated condensation of liquid bridges between
the beads. Within this picture, we are able to reproduce both the waiting-time
and humidity dependence of the ageing properties.
