520 On the two great powers, [Ocr. 
and by the new raised vapor, is more pressed within than without, 
and an enlargement of the volume of air is unavoidable in order to 
restore the equilibrium. Again, in the open air; suppose there were 
no aqueous atmosphere round the earth, only an azotic one equal to 
twenty-three inches of mercury, and an oxygenous one equal to six 
inches, ‘‘ the air being thus perfectly dry, evaporation would com- 
mence with great speed. The vapor first formed being constantly 
urged to ascend by that below, and as constantly resisted by the 
air, must in the first instance dilate the other two atmospheres, 
&e. Fie 
To this I may object, that in the experiment made on the gases 
over mercury, this liquid presses on the aqueous vapor as well as on 
the air ; and therefore both of them can re-act against it, and will de- 
press it more than either singly. But in the atmosphere the super- 
incumbent atoms of oxygen and nitrogen (which are according to the 
theory perfectly neutral towards the vapor), being the compressing 
force, cannot press on the vapor, and therefore cannot be re-acted 
against. 
Hygrometric vapor could not therefore cause the atmosphere to ex- 
pand as the vapor does the air in the experiment ; for the former va- 
por does not act on the compressing force of the atmosphere as the 
latter does on the mercury. 
The impact of the vapor against the atoms of the air, would be so 
transient and occasional (owing to the minuteness and rarity of the 
atoms), thatit is unworthy of notice as an opposition to the rise of 
the vapor. 
Again, Mr. Darron continues thus: ‘‘ At last, when all the vapor 
has ascended that the temperature will admit of, the aqueous atmos- 
phere attains an equilibrium ; it no longer presses upon the other two, 
but upon the earth ; the others return to the original density and pres- 
sure throughout.” To this I may observe ;—it is very true, that the 
others would return to their original density and pressure, but this 
is an admission which itself destroys the supposed analogy of the 
experiments, in which, while the vapor is present, the air does zot.re- 
turn to its original density. Mr. Daron continues: ‘‘ In this case 
it is true, there would not be any augmentation of volume, when 
aqueous vapor was combined with the air; humidity would increase 
the weight of the congregated atmospheres, but diminish their specific 
gravity, under a given pressure.” To this it may be replied. It is 
* Daxton’s New System of Chemical Philosophy, p. 162. 
