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themselves unable to emit any sensible amount of radiant heat, the molecules of compound gases were shown to be capable of powerfully disturbing the surrounding ether. By special modes of experiment the same was proved to hold good for the vapours of volatile liquids, the radiative power of every vapour being found proportional to its absorptive power. These experiments were based upon the fact that atoms, such, for example, as those of air, which glide through the ether without sensible resistance, cannot thus glide among the molecules of another gas. When mixed with such molecules, the heated atoms communicate their motion to the molecules by direct collision, and if these be of a complex chemical character, they instantly disturb the ether which surrounds them, and thus lose their heat. Hence the motion possessed in the first instance by the atoms, and which the atoms are incompetent to discharge directly upon the ether, may, by the intervention of more complex molecules, be thus discharged. Suppose, then, a small quantity of any vapour to be introduced into an exhausted tube, and air to be subsequently allowed to rush in and fill the tube. By its impact against the sides of the tube the air is heated; the motion of heat is instantly imparted, by collision, to the molecules of the vapour, and they in their turn impart it to the ether, or, in