36 FRAGMENTS OF SCIENCE. 



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 ele- 

 ments, 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 nitro- 

 gen 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 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 con- 

 stituting the vehicle of atomic motion, be founded in 

 fact, it is surely of interest to examine whether the vi- 

 brations of elementary bodies are modified by the act 

 of combination whether as regards radiation and ab- 

 sorption, or, in other words, whether as regards the 

 communication of motion to the ether, and the accept- 

 ance of motion from it, the deportment of the uncom- 

 bined 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 defined 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- 

 duced. But, besides the chemical unions which they 



