A proper empirical test of this hypothesis would require an extensive sifting of the
historical record that is not possible to perform here. What I can do is to mention a
ange of once successful, but (by present lights) nonreferring, theories. A fuller list will ome later, but for now we will focus on a whole family of related theories, namely,
he subtle fluids and ethers of eighteenth- and nineteenth-century physics and
hemistry.