184 Mr. Dalton on the constitution of the atmosphere, 



physical principle, that whenever two or more such gases or 

 vapours as we have been describing are put together, either 

 into a limited or unlimited space, they will finally be 

 arranged each as if it occupied the whole space, and the 

 others were not present ; the nature of the fluids and gravi- 

 tation being the only efficacious agents. 



We may now apply this doctrine in considering the earth's 

 atmosphere, on the supposition of its being in a quiescent 

 state. The gases constituting it are azotic and oxygenous 

 chiefly, a very small proportion of carbonic acid, and a small 

 proportion of aqueous vapour. If we assume the weight of 

 the atmosphere = 30 inches of mercury, and neglect the 

 carbonic acid and aqueous vapour as inconsiderable in weight, 

 we shall have ^^ of 30 = 6.3 inches for the weight of the 

 oxygenous atmosphere, and -^-^ of 30 = 23.7 inches for the 

 weight of the azotic atmosphere. For the weights of the 

 whole atmospheres on this view are proportional to the vo- 

 lumes found at the surface of the earth, and totally inde- 

 pendent of their specific gravities. The weight of the aqueous 

 vapour atmosphere is variable, and may be on an average 

 = ,4f of an inch of mercury, and that of carbonic acid = .03 

 of an inch of mercury. 



The limit of altitude in a full atmosphere (of 30 inches 

 mercury ) of oxygen gas being assumed at 45 miles, that of 

 an atmosphere of the same gas of 6.3 inches of mercury will 

 be found by calculation on the above principles to be about 

 38 miles, the atom of oxygen being 7 ; and that of azotic 

 gas of 23.7 inches weight will be found 54 miles, if the atom 

 of azote be taken as 5 ; but if the atom of azote be double 

 this weight, as is supposed by many, but I think without 



