36 FKAGMENTS OF SCIENCE. 



merely mixed, but they may be made to combine so as 

 to form molecules, each consisting 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 

 ammonia. Picturing in imagination the atoms of ele- 

 mentary bodies as little spheres, the molecules of com- 

 pound 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 examine whether the vibrations of 

 elementary bodies are modified by the act of combina- 

 tion 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 

 experiment. An experiment is well denned as a ques- 

 tion 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 be composed of elementary constituents, 

 by the compounding of which all its varieties are pro- 



