In the same way the element nitrogen combines with hydrogen, in the proportion of one atom of the former to three of the latter, to form ammonia, or spirit of hartshorn. Picturing in imagination the atoms of elementary bodies as little spheres, the molecules of compound bodies must be pictured as groups of such spheres. This is the atomic theory as Dalton conceived it. Now if this theory have any foundation in fact, and if the theory of an ether pervading space, and constituting the vehicle of atomic motion be founded in fact, we may assuredly expect the vibrations of elementary bodies to be profoundly modified by the act of combination. It is on the face of it almost certain that both as regards radiation and absorption, that is to say, both as regards the communication of motion to the ether and the acceptance of motion from it, the deportment of the uncombined will be different from that of the combined atoms.
4. Absorption of Radiant Heat by Gases.
We have now to submit these considerations to the only test by which they can be tried, namely, that of experiment. An experiment is well defined
