RADIATION 41 



this name. These atoms are endowed with powers of mu- 

 tual attraction, and under suitable circumstances they coa- 

 lesce 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 consisting 

 of two atoms of hydrogen and one of oxygen. In this con- 

 dition 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. Pict- 

 uring 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 Dal ton conceived it. Now, if this theory have 

 any foundation in fact, and if the theory of an ether per- 

 vading space, and constituting the vehicle of atomic mo- 

 tion, be founded in fact, it is surely of interest to examine 

 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 Oases 



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 question 

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

 ought to purify the question from all adjuncts which do 



