138 WRITINGS OF JOSEPH HENRY. [1855- 



soil. The separation of carbon and of hydrogen from oxygen 

 in these de-compositions is an energetic effect equivalent to 

 the heat of re-combination of those elements by combustion 

 or otherwise. The beautiful discovery of Priestley, and the 

 subsequent researches of Sennebier, De Saussure, Sir Hum- 

 phry Davy, and others, have made it quite certain that 

 those de-compositions of water and carbonic acid only take 

 place naturally in the day-time, and that light falling on the 

 green leaves, either from the sun or an artificial source, is 

 an essential condition without which they are never eflfected. 

 There cannot be a doubt but that it is the dynamical energy 

 of the luminiferous vibrations which is here efficient in forc- 

 ing the particles of carbon and hydrogen away from those of 

 oxygen, towards which they are attracted with such powerful 

 affinities, and that luminiferous motions are reduced to rest 

 to an extent exactly equivalent to the potential energy thus 

 called into being. Wood fires give us heat and light which 

 have been got from the sun a few years ago. Our coal fires 

 and gas lamps bring out, for our present comfort, heat and 

 light of a primeval sun, which have lain dormant as a 

 potential energy beneath the seas and mountains for countless 

 ages." (Prof. William Thomson.) 



A striking example of the transformation, as it were, of 

 the force of motion into heat is exhibited by an article of 

 apparatus now in the cabinet of the Smithsonian Institution 

 and devised by M. Leon Foucault, of Paris. Between the 

 poles of a strong electrio-magnet a heavy metallic disc is 

 made to rotate, and although the revolving body does not 

 touch the magnet, yet its motion is stopped by it in a few 

 seconds. The momentum of the disc which is thus over- 

 come gives rise to heat; for the re-action of the magnet pro- 

 duces a current of electricity, and in the resistance to this 

 the heat is generated. A body in motion is in a state of 

 power, and it cannot come to rest without producing some 

 effect on the surrounding matter. The ultimate effect in 

 this case is an agitation of the atoms of the metal. 



Condition of the Earth in Space. 



Having given a general view of the atomic theory in its 

 widest generalizations, we now propose to consider its appli- 

 tion to the physical phenomena of our globe. For this pur- 



