160 WRITINGS OF JOSEPH HENRY. [1855- 



Secondly. We may suppose that the two classes of atoms 

 repel each other, but in this case no mixture could take place; 

 the heavier gas would remain in the lower vessel, while the 

 lighter one would occupy the upper position. 



Thirdly. If we suppose the atoms of the two gases have 

 no action on each other, but are free to obey their own re- 

 pulsions, then the atoms of each gas will expand into the 

 void space of the interstices of the other, and the diffusion 

 indicated by the experiment will be produced. 



It follows from this hypothesis that the bulk of the mix- 

 ture should remain the same before and after the mingling 

 takes place. Let us suppose each vessel to contain a foot of 

 gas, and that the repulsive energy is sufficient to sustain a 

 weight of 15 pounds to the square inch ; and let us suppose 

 the interior of the vessel containing the hydrogen is a vacuum. 

 Then it is evident that the oxygen in the lower vessel, being 

 relieved from the pressure of the atmosphere, will expand 

 and fill both vessels, and by the law of Mariotte, its elastic 

 force or repulsive energy will be reduced to one-half or 7J 

 pounds to the square inch. The same will take place with 

 regard to the hydrogen. It will expand downward and fill 

 both vessels, and its elastic force will be reduced to one-half 

 or to 7^ pounds to the square inch. If therefore the gases 

 are vacuums to each other, they will each expand into the 

 other and form a mixture of two gases, the pressure of each 

 of which against the sides of the vessel will be 7| pounds 

 to the square inch, and consequently the whole pressure will 

 be 15 pounds. 



The theory of Dalton is in exact accordance with all the 

 facts, though it may be difficult to conceive of atoms, such 

 as those of oxygen and hydrogen, as being without action 

 on each other particularly when highly compressed. In- 

 deed, Mr. Dalton in the latter part of his life was inclined 

 to refer this seeming want of repulsion to the fact of the dif- 

 ferent sizes of the atoms, or in other words to the difference in 

 the spheres of their repulsive energies. If two classes of 

 atoms were thus mingled with each other, it is evident that 

 they could not be in equilibrium until the one was generally 



