26 PR A OMENTS OF SCIENCE. 



mutual attraction, and under suitable circumstances they 

 coalesce to form compounds. Thus oxygen and hydrogen 

 are elements when separate, or merely mixed, but they 

 may be made to combine so as to form molecules, each con- 

 sisting of two atoms of hydrogen and one of oxygen. In 

 this condition they constitute water. So also chlorine and 

 sodium are elements, the former a pungent gas, the latter 

 a soft metal; and they unite together to form chloride of 

 sodium or common salt. 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 am- 

 monia. 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, it is surely of interest to ex- 

 amine whether the vibrations of elementary bodies are 

 modified by the act of combination whether as regards 

 radiation and absorption, or, in other words, whether as 

 regards the communication of motion to the ether, and the 

 acceptance of motion from it, the deportment of the 

 uncombined atoms will be different from that of the 

 combined. 



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 experi- 

 ment. An experiment is well defined as a question put to 

 Nature; but, to avoid the risk of asking amiss, we ought 

 to purify the question from all adjuncts which do not 

 necessarily belong to it. Matter has been shown to bo 

 composed of elementary constituents, by the compound inir 

 of which all its varieties are produced. But, besides the 

 chemical unions which they form, both elementary and 

 compound bodies can unite in another and less intimate 

 way. Gases and vapors aggregate to liquids and solids, 

 without any change of their chemical nature. We do not 

 yet know how the transmission of radiant heat may be 

 affected by the entanglement due to cohesion; and, as our 

 object now is to examine the influence of chemical union 

 alone, we shall render our experiments more pure by liber- 



