-1859] WRITINGS OF JOSEPH HENRY. 95 



rod of transparent glass, to look into the interior of the elastic 

 body and observe the changes there produced. 



The difference between the compressibility of air and 

 of steel depends upon the difference in the repulsion of the 

 atoms in the two cases. But in the latter, as well as in the 

 former, there is the most delicate balance of forces; for though 

 a bar of good steel resists the weight of 60,000 pounds to the 

 square inch tending to separate it in the direction of its 

 length, yet the atoms may be thrown into vibration by the 

 minutest force; and this is the case with all solids. A single 

 tap with the end of a penknife on the table of the large lec- 

 ture room of the Smithsonian Institution is sufficient not 

 only to throw into vibration every particle of air in the room, 

 but also every particle of the solid parts of the edifice. The 

 agitation of the air is proved by the sound, discernible in 

 every part of the room, and the vibrations of the solid parts 

 also by the transmission of sonorous waves with even less 

 loss than in the air. 



The repulsion of which we have spoken, and which takes 

 place only at minute distances, though these may be exceed- 

 ingly great when measured by the size of the atoms, appears 

 to be an essential endowment of matter, and is exhibited as 

 well between the atoms of the sDtherial medium as between 

 those of air and other grosser assemblages of matter. 



All bodies (as a general rule) are enlarged by an increase 

 of temperature. But this result, as we shall endeavor to show, 

 is not from an increase of the original repulsion, but from 

 an energetic vibration imparted to the atoms, which tends to 

 separate them and produce the phenomena improperly as- 

 cribed to an imaginary fluid called heat. 



The medium of radiation. — We are obliged to assign to the 

 setherial medium a similar constitution to that possessed by 

 grosser matter; namely, that it consists of inert atoms at 

 great distances from each other relative to their own size, 

 and each kept in position by attracting and repelling forces. 

 Through this medium impulses or minute agitations are 

 transmitted in celestial space, from planet to planet, and from 

 system to system, which tremors or waves constitute light. 



