THE MOLECULAR COMPOSITION OF MATTER 79 



advanced in an acceptable form until the work of 

 Clausius in 1857. In Dalton's day the effect which 

 Boyle had noticed was explained on the basis of forces. 

 This was largely, of course, because of the influence 

 of Newton's concept of forces. 1 For the present it is 

 only necessary to note that the pressure was supposed 

 to be due to the interactions of the various corpuscles. 

 These were said to repel each other with a force which 

 became smaller as the separation between the particles 

 increased. If both kinds of particles repelled then 

 stratification was still possible. But if the only actions 

 were between similar particles then each gas would 

 tend to expand and fill any available space. Dal ton, 

 therefore, assumed that each gas acted as a vacuum 

 so far as the other gas was concerned. 



This, as we shall see, is also the modern view, but we 

 recognize to-day that it is only approximately true and 

 depends upon the actual amounts of the gases as com- 

 pared to the volume of the container. If, compared to 

 this latter volume, the space occupied by the gas mole- 

 cules is small, then their motions are practically in- 

 dependent of each other's presence. The phenomenon 

 has a parallel in the case of a dancing-floor. If the 

 space occupied by the couples while at rest is small as 

 compared to the floor area, then* motions hi dancing 

 will be uninfluenced by each other's presence. 



For pressures not greater than a few atmospheres 

 Dalton's assumption would be true. He verified it 

 experimentally and announced his law of partial 

 pressures, that the pressure exerted by a contained 



1 The laws of Newton and the modern theory as to gas pressures 

 will be considered in Chapters XII and XIII, respectively. 



