Jeans' formula gives the condition that a gravitating mass of gas shall be stable to small fluctuations in the density. It was originally obtained for a static mass of uniform density, and the proof is questionable. In this paper the formula is derived by studying small perturbations of the density and velocity in an expanding Newtonian world-model. It turns out that, given a suitable interpretation of the quantities involved, the formula is valid in all cosmologies except the steady-state theory. Although instability occurs in the models as suggested by Jeans' formula, the condensation process is too slow to account for the formation of the nebulae–a result which has now been reached in several ways, and which may be regarded as well-established. In the Newtonian version of the steady-state theory small perturbations of the average density die out, and so Jeans' formula does not apply. The relevance of this to the work of Sciama, who assumes the contrary, is discussed. It seems that the claim that the steady-state theory explains the formation of the nebulae requires further substantiation.