160 THE HISTORY OF SCIENCE 



He made a diagram in which one atom of oxygen was 

 combined with one atom of nitrogen, but in this case 

 the oxygen was insufficient to satisfy all the nitrogen 

 of the atmosphere. If the air was made up partly of 

 pure nitrogen, partly of a compound of nitrogen and 

 oxygen, and partly of a compound of nitrogen, oxy- 

 gen, and aqueous vapor, then the triple compound, as 

 heaviest, would collect toward the surface of the earth, 

 and the double compound and the simple substance 

 would form two strata above. If to the compounds 

 heat were added in the hope of producing an un- 

 stratified mixture, the atmosphere would acquire the 

 specific gravity of nitrogen gas. "In short," says 

 Dalton, " I was obliged to abandon the hypothesis 

 of the chemical constitution of the atmosphere alto- 

 gether as irreconcilable to the phenomena." 



He had to return to the conception of the indi- 

 vidual particles of oxygen, nitrogen, and water, each 

 a center of repulsion. Still he could not explain why 

 the oxygen did not gravitate to the lowest place, the 

 nitrogen form a stratum above, and the aqueous vapor 

 swim upon the top. In 1801, however, Dalton hit 

 upon the idea that gases act as vacua for one another, 

 that it is only like particles which repel each other, 

 atoms of oxygen repelling atoms of oxygen and atoms 

 of nitrogen repelling atoms of nitrogen when these 

 gases are intermingled in the atmosphere just as 

 they would if existing in an unmixed state. " Accord- 

 ing to this, we were to suppose that atoms of one kind 

 did not repel the atoms of another kind, but only 

 those of their own kind." A mixed atmosphere is as 

 free from stratifications, as though it were really 

 homogeneous. 



