174 FKAGMENTS OF SCIENCE. 



of heat arc in this way reducible to interchanges of motion ; 

 and it is purely as the recipients or the donors of this mo- 

 tion, that we ourselves become conscious of the action of 

 heat and cold. 



3. The Atomic Theory in reference to the Ether. 



The word " atoms " has been more than once employed 

 in this discourse. Chemists have taught us that all matter 

 is reducible to certain elementary forms to which they give 

 this name. These atoms are endowed with powers of 

 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 consisting 

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

 dition they constitute w T ater. 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 or spirit 

 of hartshorn. Picturing in imagination the atoms of ele- 

 mentary 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 deport- 

 ment of the uncombined will be different from that of the 

 combined atoms. 



