II. _The Carbon Dioxide Theory._ At about the time that the eccentricity theory was being relegated to a minor niche, a new theory was being developed which soon exerted a profound influence upon geological thought. Chamberlin,[11] adopting an idea suggested by Tyndall, fired the imagination of geologists by his skillful exposition of the part played by carbon dioxide in causing climatic changes. Today this theory is probably more widely accepted than any other. We have already seen that the amount of carbon dioxide gas in the atmosphere has a decided climatic importance. Moreover, there can be little doubt that the amount of that gas in the atmosphere varies from age to age in response to the extent to which it is set free by volcanoes, consumed by plants, combined with rocks in the process of weathering, dissolved in the ocean or locked up in the form of coal and limestone. The main question is whether such variations can produce changes so rapid as glacial epochs and historical pulsations. Abundant evidence seems to show that the degree to which the air can be warmed by carbon dioxide is sharply limited. Humphreys, in his excellent book on the _Physics of the Air_, calculates that a layer of carbon dioxide forty centimeters thick has practically as much blanketing effect as a layer indefinitely thicker. In other words, forty centimeters of carbon dioxide, while having no appreciable effect on sunlight coming toward the earth, would filter out and thus retain in the atmosphere all the outgoing terrestrial heat that carbon dioxide is capable of absorbing. Adding more would be like adding another filter when the one in operation has already done all that that particular kind of filter is capable of doing. According to Humphreys' calculations, a doubling of the carbon dioxide in the air would in itself raise the average temperature about 1.3 deg.C. and further carbon dioxide would have practically no effect. Reducing the present supply by half would reduce the temperature by essentially the same amount.