96 WRITINGS OF JOSEPH HENRY. [1855- 



heat, and other emanations received by us from the sun. 

 That is to say, the solar emanations are not matter, but motion 

 communicated from atom to atom, beginning at the lumi- 

 nous body and diffused in widening spherical surfaces, en- 

 larging in size and diminishing in intensity to the farthest 

 conceivable portion of space. 



The atoms of the setherial medium are assumed to be 

 perfectly free to move in all directions so that the earth 

 and denser bodies experience no retardation as yet measur- 

 able; though lighter bodies, such as comets, apparently ex- 

 hibit an effect of this kind for the same reason that a flock 

 of cotton is more retarded in falling through the air than 

 a piece of lead. At first sight it might appear paradoxical 

 that atoms, which are kept in position by powerful attraction 

 and repulsion, should yet be perfectly movable among each 

 other; but this condition is observed in liquid water, the 

 particles of which, though they exhibit perfect mobiiit}'-, yet 

 repel and attract each other with immense force. This arises 

 from the fact that every atom beneath the surface of a fluid 

 is equally attracted and repelled on all sides by the surround- 

 ing atoms, and is therefore perfectly free to move. Not so 

 however with the atoms at the surface, for they are attracted 

 downwards without a counteracting force to attract them 

 upwards, and hence great resistance is manifested when we 

 attempt to separate them. 



The author of this essay has shown from conclusive experi- 

 ments that the attraction of water for water is as great as that 

 of ice for ice,* and that the difference of the two conditions 

 consists in the perfect mobility of the atoms in the former 

 case, and not in the neutralization of cohesion, as is gener- 

 ally supposed. If we attempt to draw up from the surface 

 of water a circular disc of metal, say of an inch in diameter, 

 we shall see that the water will adhere and be supported 

 several lines above the general surface. This adhesion, on 

 account of the perfect mobility of the atoms, is due alone to 

 the attraction of the atoms of the external film and not to 

 those of the whole mass which is elevated. This experi- 



£* Proceedings of American Philosophical Society. Seea7ite, vol. i, p. 217.] 



