The absorption of many gases and vapours having been determined, their radiation was measured by a very simple arrangement. The thermo-electric pile was raised on a stand with a screen of polished tin in front of it. A heated copper ball in a perforated ring on a low stand was placed behind the screen; all direct radiation from the ball was thus cut off, but the heated air rising in a column above the screen radiated its heat on the pile and deflected the needle of the goniometer 60° when the ball was red-hot; but the radiation of the hot air was neutralized by another source of radiant heat on the opposite side of the pile which kept the needle steadily at zero. Then a purified gas or vapour conveyed by a pipe into the perforated ring which held the ball rose mixed with the heated air above the screen, but the radiation of the gas or vapour alone was shown by the deflection of the needle, because that of the air was compensated. With this apparatus Professor Tyndall proved that the amount of the absorption of each gas and vapour is exactly equal to the amount of its radiation. He has shown that this result is a necessary consequence of the dynamical nature of heat. For as no atom or molecule is capable of existing in vibrating ether without accepting a portion of the motion, the very same quality whatever it may be that enables it to do so, must enable it to impart its motion to still ether when plunged into it. ‘Hence from the existence of absorption we may on theoretic grounds infallibly infer a capacity for radiation; from the existence of radiation we may with equal certainty infer a capacity for absorption, and each of them must be regarded as the measure of the other.’ This reasoning, founded simply on the mechanical relations of the ether and the atoms immersed in it, is completely verified by experiment.