Page:Popular Science Monthly Volume 29.djvu/376

362 than it now is. All conclusions, however, in regard to the sun's former temperature must be hypothetical; but, if it be a gaseous body, as suggested by Professor Young, it has been growing hotter all the time it has been giving out heat. To this, as to all other theories heretofore advanced, there lies the serious objection that they ignore the world-wide uniformity of light and actinic force, and no theory that fails here can be satisfactory.

A perpendicular axis alone does not account for the warmth in polar regions. On the contrary, with such an axis, they would receive during the year less heat than they now do, and hence Dr. Croll infers that a perpendicular axis would make the polar climate less genial. This is true, if temperature depends solely on the amount of heat received. But, as every one knows, it depends far more upon the amount retained. Green-houses and drying-houses are often uncomfortably warm when the mercury without indicates a temperature near freezing. The solar rays readily enter through the glass, and are absorbed by the floors, walls, etc., while the heat radiated back is unable to escape. # Many substances possess this property, and Professor Tyndall has shown that among them are carbonic acid and aqueous vapor.

In the present state of our knowledge, it is impossible to form even an approximate estimate as to the actual amount of carbon stored in the earth's crust as graphite, coal, lignite, bitumen, petroleum, etc., but it must be many times—probably many hundred times—greater than that now remaining in the atmosphere. All these forms of carbon are directly or indirectly of vegetable origin, and hence it once existed in the form of carbonic acid. It has been said, however, that so much CO2 was not found in the air at any one time, but that it was given out by volcanoes just about fast enough to take the place of that which was stored in the earth's crust. But, as Professor Dana remarks, volcanoes do not originate this gas; they give it out only as their fires come in contact with limestone, and this occurs but rarely now, and was still more uncommon in Palæozoic times.

It appears, therefore, that at some remote period all the carbon which has since been a portion of animal or vegetable forms, existed as free carbonic acid, and formed a part of the then atmosphere. With the beginning of plant-life a process of elimination commenced. It continued till about the close of the Tertiary, when the amount taken out by living forms and that restored to the atmosphere by decomposition became equal—a condition which still exists.

The CO2 is now diffused with great uniformity over the earth, and,