The method of experiment here pursued, though not of the simplest character, is still easy to grasp. When air is permitted to rush into an exhausted tube, the temperature of the air is raised to a degree equivalent to the _vis viva_ extinguished. [Footnote: See above for a definition of _vis viva_.] Such air is said to be dynamically heated, and, if pure, it shows itself incompetent to radiate, even when a rock-salt window is provided for the passage of its rays. But if instead of being empty the tube contain a small quantity of vapour, the warmed air communicates its heat by contact to the vapour, the molecules of which convert into the radiant form the heat imparted to them by the atoms of the air. By this process also, which I have called Dynamic Radiation, the reciprocity of radiation and absorption has been conclusively proved. [Footnote: When heated air imparts its motion to another gas or vapour, the transference of heat is accompanied by a change of vibrating period. The Dynamic Radiation of vapours is rendered possible by this transmutation of vibrations.] In the excellent researches of Leslie, De la Provostaye and Detains, and Balfour Stewart, the same reciprocity, as regards solid bodies, has been variously illustrated; while the labours, theoretical and experimental, of Kirchhoff have given this subject a wonderful expansion, and enriched it by applications of the highest kind. To their results are now to be added the foregoing, whereby gases and vapours, which have been hitherto thought inaccessible to experiments with the thermo-electric pile, are proved by it to exhibit the indissoluble duality of radiation and absorption, the influence of chemical combination on both being exhibited in the most decisive and extraordinary way. ******************** 15. Influence of Vibrating Period and Molecular Form. Physical Analysis of the Human Breath.