  We clarify the procedure for expressing the Friedmann equation in terms of
directly measurable cosmological scalars constructed out of higher derivatives
of the scale factor. We carry out this procedure for pure dust, Chaplygin gas
and generalised Chaplygin gas energy-momentum tensors. In each case it leads to
a constraint on the scalars thus giving rise to a test of General Relativity.
We also discuss a formulation of the Friedmann equation as unparametrised
geodesic motion and its connection with the Lagrangian treatment of perfect
fluids coupled to gravity.
