78 THE REALITIES OF MODERN SCIENCE 



clew to its physical structure. The union of oxygen 

 and nitrogen was supposed by many scientists to be 

 chemical, one substance dissolving the other. When 

 water evaporated into the air it, also, was supposed to 

 be dissolved by the existing compound of nitrogen and 

 oxygen. This idea was incomprehensible to Dalton. 

 He tried to visualize such a mixture in terms of small 

 particles which he called atoms, adopting the ideas 

 of a granular structure. The difficulty in his mind 

 was that the compound particles would be of different 

 masses and should ultimately settle toward the surface 

 of the earth, resulting in a stratified atmosphere just 

 as two liquids of different density will separate under 

 gravity. It had been found, however, by testing air, 

 obtained at various altitudes, that the proportions of 

 oxygen and nitrogen were essentially constant and hence 

 that no such phenomenon of stratification occurred. 

 Such a chemical theory could not be reconciled with 

 the observed facts, and yet Dalton felt the need of 

 picturing the phenomenon in terms of corpuscles. At 

 first it seemed to him equally inconceivable that the 

 air should be merely a mechanical mixture of particles 

 of the two gases just as we might make a mixture of 

 baseballs and tennis balls. 



The pressure of a contained gas had been shown by 

 Boyle to be due to its " spring' 7 or elasticity. Ber- 

 nouilli in 1738 had suggested that the cause of the 

 pressure was the impacts upon the surface of the con- 

 tainer of the small particles of the air. The kinetic 

 theory, which explains such pressure as due to molecu- 

 lar impacts and the motions of the molecules as due 

 to the energy which they possess, was not, however, 



