Now, it seems to us that there is quite as much of hypothesis in this speculation of Laplace as in the undulatory theory of light. This atmosphere of the sun extending to the utmost limits of our planetary system! What proof have we that it ever existed? what possible grounds have we for believing, what motive even for imagining such a thing, but the very same description of proof given and rejected for the existence of a luminiferous ether--namely, that it enables us to explain certain events supposed to result from it? Nor is the thing here imagined any the less a novelty, because it bears the old name of an atmosphere. An atmosphere containing in itself all the various materials which compose our earth, and whatever else may enter into the composition of the other planets, is as violent a supposition as an ether, not perceptible to the senses except by its influence on the nerves of sight. And this cooling down of the sun! What fact in our experience enables us to advance such a supposition? We might as well say that the sun was getting hotter every year, or harder or softer, or larger or smaller. Surely Mr Mill could not have been serious when he says, that "the known laws of matter authorize us to suppose, that a body which is constantly _giving out so large an amount of heat_ as the sun is, must be progressively cooling"--knowing, as we do, as little how the sun occasions heat as how it produces light. Neither can it be contended that because no absolutely new substance, or new property of matter, is introduced, but a fantastic conception is framed out of known substances and known properties, that therefore there is less of rash conjecture in the supposition. In fine, it must be felt by every one who reads the account of this speculation of Laplace, that the only evidence which produces the least effect upon his mind, is the corroboration which it receives from the calculations of the mathematician--a species of proof which Mr Mill himself would not estimate very highly.