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 is altogether under the mark; and that we are indebted to this wonderful substance, to an extent not accurately determined, but certainly far beyond what has hitherto been imagined, for the temperature now existing at the surface of the globe.

14. Reciprocity of Radiation and Absorption.

Throughout the reflections which have hitherto occupied us, the image before the mind has been that of a radiant source generating calorific waves, which on passing among the scattered molecules of a gas or vapour were intercepted by those molecules in various degrees. In all cases it was the transference of motion from the ether to the comparatively quiescent molecules of the gas or vapour. We have now to change the form of our conception, and to figure these molecules not as absorbers but as radiators, not as the recipients but as the originators of wave motion. That is to say, we must figure them vibrating and generating in the surrounding ether undulations which speed through it with the velocity of light. Our object now is to enquire whether the act of chemical combination, which proves