Page:Popular Science Monthly Volume 29.djvu/377

Rh for lack of reason to the contrary, we must believe that it was equally uniform in geological times. It acted as glass does in a green-house; it retained the heat radiated from the earth's surface, and consequently caused a rise in temperature. This increased in a higher ratio the capacity of the air for water, and that in its turn aided still further in retaining the heat, and of course made the climate warmer. In this, I think, lies the secret of the warm climate in high latitudes in those early times, the otherwise cold polar regions being protected by this double "blanket." The effect of such a covering is well set forth by Professor Tyndall in "Heat considered as a Mode of Motion," pp. 405, 406. I quote only one sentence: "The removal, for a single summer night, of the aqueous vapor which covers England would be attended by the destruction of every plant which a freezing temperature could kill."

In contrast with this, I add one illustration of the temperature possible were the earth covered with a "warm blanket" equal in heat-retaining power to glass. I quote from Professor Langley's summary of work on Mount Whitney to ascertain the amount of heat the sun sends to the earth: "On the summit of Mount Whitney the temperature in a blackened copper vessel, covered by two sheets of common window-glass, rose above the boiling point. With such a vessel water could be boiled in the snow-fields of Mount Whitney by the direct solar rays."

Besides carbonic acid and water, there probably were in the early atmosphere other gases and vapors. Ammonia would produce thirteen times the effect of CO2 at the same density, and marsh-gas four and one half times, and so of others ("Heat as a Mode of Motion," p. 362). Whatever there was of these, their influence tended to increase the "warm blanket." The amazingly slow change of temperature in the early periods finds a reasonable explanation in the effect of those gases and vapors in the atmosphere.

Professor Tyndall has shown that, commencing with a vacuum, and adding a small number of very small increments, the absorption is sensibly proportional to the increments, but, as the quantity increases, the deviation from proportionality augments (idem, p. 356); at length a condition is reached in which further increments produce very little effect. The converse must also be true. Commencing with a great amount of the gas, or vapor, a very great number of decrements will be needed to produce any sensible effect; then a smaller number, and so on, until toward the end, and then the decrement needed will be very small, and the effect comparatively large.

The following diagram, made from a table on page 35 of the same work, shows this more clearly. The curved line indicates temperature for equal increments of the gas.

The diagram is for sulphuric ether and olefiant gas. All other (compound) gases and vapors observe the same law, but differ in the