forms, is insignificant when compared with that which has been absorbed during the decomposition of silicated rocks and is now fixed in the form of limestones. The magnitude of this process is seen when we consider that all the argillaceous rocks and clays of the stratified formations have come from the decay of the feldspars and other silicates of the earlier eozoic terrenes through the intervention of carbonic acid, and that the resulting alkaline and earthy carbonates are now represented by the limestones so abundant in the earth's crust. It was shown, in the author's communication already quoted, that a layer of pure limestone covering the earth's surface to a depth of about twenty-eight feet (8.61 metres) corresponds to an amount of carbonic acid which, if set free, would double the weight of the present atmosphere, and the existence of great limestone and dolomite formations, many hundred feet in thickness, at different geological horizons over wide areas, will, it is believed, justify the conclusion that the earth's crust contains, fixed in the form of carbonates, an amount of carbonic acid which, if liberated in a gaseous form, would be equal in weight to at least two hundred atmospheres like the present one. A portion of this carbonic acid was doubtless separated at an early period in the history of our globe, since the limestones of eozoic rocks are of considerable thickness, and those of more recent times are in part derived from the solution and redeposition of the older limestones. The only known source of carbonic acid, apart from combustion and respiration, are certain terrestrial exhalations of the gas, probably due to chemical reactions liberating small portions which had long before been fixed in the form of carbonates. We are thus forced to one of two conclusions: either the wholly improbable one that the atmosphere, since the appearance of organic life on the earth, has been one of nearly pure, carbonic acid, and of such immense extent that the pressure at the surface would have sufficed, at ordinary temperatures, for its liquefaction; or else, the atmosphere being so constituted as to permit vital processes, that carbonic acid, as fast as removed by chemical action at the earth's surface, was supplied from some extra-terrestrial source. We may, in accordance with this last hypothesis, admit that the atmosphere is not terrestrial but cosmical, and that the air, together with the water surrounding our globe (whether in a liquid or a vaporous state), belongs to a common elastic medium, which, extending throughout the interstellary spaces, is condensed around attracting bodies in amounts proportional to their mass and temperature, while in the regions most distant from these centres of attraction this universal atmosphere would exist in the state of greatest tenuity. Such being the case, a change in the atmosphere of any globe, whether by the absorption or disengagement of any gas or vapor, would, by the laws of diffusion and of static equilibrium, be felt everywhere throughout the universe; and the fixation of carbonic acid at the surface of our planet would not only bring in a supply of this gas from the worlds beyond, but, by reducing the total amount of it in the universal atmosphere, would diminish the atmospheric pressure at the surface of our own and of other worlds. "This hypothesis is not altogether new. Sir William R. Grove, in 1842, put forth the notion that the medium of heat and light may be 'a universally diffused matter,' and, subsequently, in 1843, in his celebrated 'Essay on the Correlation of Physical Forces,' in the chapter on Light, concludes, with regard to the atmosphere of the sun and the planets, that there is no reason why these atmospheres should not be, with reference to each other, in a state of equilibrium. Ether, which term we may apply to the highly-attenuated matter existing in the interplanetary spaces, being an expansion of some or all of these atmospheres, or of the more volatile portions of them, would thus furnish matter for the transmission of the modes of motion which we call light, heat, etc., and possiby minute portions of these atmospheres may, by gradual accretions and subtractions, pass from planet to planet, forming a link of material communication between the distant monads of the universe.' Subsequently, in his address as President of the British Association for the Advancement of Science, in 1866, Grove further suggested that this diffused matter might be a source of solar heat, inasmuch as the sun may 'condense gaseous matter as it travels in space, and so heat may be produced.' "