-1859] WRITINGS OF JOSEPH HENRY. 159 



thus a reduction of sensible heat will be observed as we as- 

 cend in the atmosphere. The equilibrium we have described 

 would not however be a stable one, and hence the upper 

 strata of the atmosphere contain .more heat per pound than 

 the lower. 



Until about the middle of the last century, the atmosphere 

 was supposed to consist of one simple homogeneous substance, 

 and after modern chemistry had discovered it to be a com- 

 pound, the ingredients were thought to be chemically united. 

 It was also supposed, until the researches of Dalton proved 

 the contrary, that the vapor of water found in the atmos- 

 phere was dissolved in it, as one liquid is dissolved in 

 another. 



Dalton was the first to advance the proposition that the 

 atoms of different gases neither attract nor repel each other ; 

 and though each offers a slight mechanical obstruction to 

 the free motion of the other, yet if sufficient time be allowed, 

 each will arrange itself as if the other did not exist ; or in 

 other words while the atoms of the same gas repel one 

 another, those of different gases exert no action of this kind, 

 and are in fact statical though not dynamical vacuums each 

 to the other. The fundamental fact on which this theory 

 is based is the following: If two wide-mouthed jars be 

 placed, one on the other, mouth to mouth, the lower one 

 being filled with oxygen or heavy gas, and the upper one 

 with hydrogen, the lightest of all gases, and thus suffered 

 to remain, after a short time it will be found that the two 

 gases will be thoroughly mingled through both jars; the 

 light gas will descend and mix with the heavier, while the 

 heavier will in turn ascend and mix with the lighter. 

 There will be no increase or diminution of bulk of the two 

 gases after they have thus mingled. In order to explain 

 the mixing of gases, three hypotheses may be assumed : 



First. We may suppose that the atoms have an affinity 

 for each other in their gaseous state. But if this were the 

 case, from general analogy there should be a diminution of 

 the bulk ; the number of centres of repulsion would be 

 diminished, and also the intensity of the action of each 

 would be at least partly neutralized. 



