Page:Popular Science Monthly Volume 13.djvu/259

Rh But our dull imaginations have too much in common with the sluggish physical movements of our island-home for us to soar to the heights of calculation which seem so easy to Mr. Southall.

"It is in dealing with the age of the great extinct mammals that our author shows most conspicuously this tendency to shirk (unconsciously, of course) the difficulties with which the problem is surrounded in Europe. That early man was contemporary—in what is now England, Southern France, and Germany—with the lion, the bear, the hyena, the gigantic elk, the reindeer, and the mammoth, the conditions under which their bones are found intermingled have long placed beyond reasonable doubt. In the case of the reindeer and the mammoth, the evidence is raised to certainty by the discovery of outlines of those animals etched, with rude but highly-expressive art, upon fragments of bone."

noticed this admirable volume upon its first appearance three or four years ago, and are glad to observe that it has passed to a second edition. The plan of the work is not changed, as its essays have something of an historical import, which it was thought inexpedient to disturb, so that in the work of revision the author has confined himself to the correction of typographical errors in the text. But he has prefixed to the volume an elaborate essay of very great interest upon questions connected with the general scope of the work, upon which decided progress has been made since the first publication, and these additions are well worth the price of the new edition. We quote a portion of this preliminary essay, which treats of the ancient constitution of the air, and from which the author rises to the consideration of cosmical atmospheres and the diffused medium of celestial space:

"On pages 46-48 is a suggestion, made many years since, regarding the question of the temperature of the earth's surface in former geological periods, which, from its bearings, both direct and indirect, on some recent geological theories, calls for further notice. From the great amount of carbon and hydrocarbons of organic origin found in the rocky strata of the earth, it has long been inferred that the atmosphere of earlier times must have contained a large quantity of carbonic dioxide (carbonic acid) which yielded up its carbon for the nutrition of the ancient floras. From this the late Major Edwin B. Hunt concluded that, the atmosphere in former periods being much denser than at present, the temperature at the earth's surface, due to solar radiation, would be greater than now. It was subsequently pointed out by the present writer that, as already shown by Tyndall, the relations of carbonic acid to radiant heat are such that a quantity of this gas too small to affect considerably the weight of the atmospheric column would, by preventing the loss of heat, suffice to produce a tropical temperature over the earth at the sea-level.

"The quantity of carbon which has been removed from the air by vegetation in past ages is, however, very considerable. In a communication by the writer to the American Association for the Advancement of Science, at Buffalo, in 1866, it was stated that the whole amount of free oxygen in the present atmosphere is no more than sufficient to form carbonic acid with the carbon of a layer of coal covering the globe one metre in thickness, and that the aggregate of carbonaceous matter in the earth's crust would probably much exceed this. Such a layer of coal, of specific gravity 1.25, would have a weight equal to 3,160,000 gross tons to the square mile; while Mr. J. L. Mott, in a communication to the British Association for the Advancement of Science, in 1877, estimates the total amount of carbonaceous substances in the earth at not less than 3,000,000 tons of carbon to the square mile, and probably many times greater. This minimum amount of pure carbon is equal to 600 times the present amount of carbonic acid in the atmosphere, or to nearly one-fourth its entire volume; and, inasmuch as the fixation of carbon by vegetation liberates a corresponding volume of oxygen, would represent, according to him, a greater amount of this gas than the present atmosphere contains. In addition to this, it must be considered that the composition of the various hydrocarbonaceous minerals shows a deoxidation not only of carbonic acid but of water. The amount of liberated oxygen derived from water equals, for the various coals and asphalts, from one-eighth to one-fourth, and for the petroleums one-half of that set free in the deoxidation of the carbon which these hydrocarbonaceous bodies contain. To this must be added also the oxygen set free in the generation of metallic sulphides by the deoxidation of sulphates, which is effected through the agency of organic matters, and indirectly liberates oxygen. Against this we must, however, set the unknown but very considerable amount of oxygen absorbed in the peroxidation of ferrous oxide liberated in the decay of the silicates of crystalline rocks; which may, perhaps, serve to explain the disappearance from the air of the whole of this excess of oxygen.

"The terrestrial vegetation and the air-breathing fauna, which we find from Palæozoic ages, are, it is unnecessary to remark, incompatible with an atmosphere holding one-fourth its volume of carbonic acid, and the difficulty of the problem is greatly increased when we consider that this amount, corresponding to the carbon fixed in the earth's crust in deoxidized