02 WRITINGS OF JOSEPH HENRY, [1855- 



other by means of the force of gravitation, and also by tre- 

 mors or vibrations in this medium, radiating in every direc- 

 tion from each body as a centre. 



The atoms of matter are thus separated by intervals ; 

 and before we proceed further it will be necessary to con- 

 sider more particularly this separation. It must be recol- 

 lected that the hypothesis we are presenting is not the mere 

 creature of the imagination, but is based upon a generaliza- 

 tion of actual observation on the different states of grosser 

 matter. We shall therefore commence with the consideration 

 (as an example) of the constitution of the air. This we assume 

 to consist of atoms, each endowed with attracting and repel- 

 ling forces. That these atoms are not in contact with each 

 other, will be evident from the fact that if we apply a suffi- 

 cient pressure to a quantity of air taken at its greatest known 

 rarity, it may be compressed into at least one ten-thousandth 

 part of its primitive volume. The sum of the magnitudes 

 of the void spaces is therefore, in this case, at least ten thou- 

 sand times greater than the sum of the material parts, what- 

 ever be their nature. In order to explain this we are ob- 

 liged to suppose that each atom is endowed with a repul- 

 sive force similar to that possessed by one pole of a magnet for 

 a similar pole of another magnet. And this repulsion in- 

 creases with the diminution of distance between the atoms. 

 It is feeble when the volume of air is expanded to its fullest 

 extent, and exceedingly powerful when highly compressed. 

 Whatever weight we may put on the top of a piston fitted to a 

 cylinder filled with air will be sustained by the repulsion 

 of the atoms. The piston will descend until each atom is 

 brought precisely to that state of proximity to the next that 

 the repulsive energy between the atoms just balances the 

 weight on the piston, and thus the most delicate equipoise 

 is afforded by the air. The slightest extraneous force is suf- 

 ficient to disturb the equilibrium, which is again restored by 

 a series of decreasing oscillations. 



If the atoms of the air however are removed to a much 

 greater distance, the repulsion entirely ceases, and attraction 

 of gravitation takes its place. If it were not for this, the 



