6 . PRESSURE OF GASES 13 



Bernoulli supposed a mass of gas to be confined in a 

 vessel with a movable but air-tight lid, such as the cylinder 

 of an air-pump ; this gas can be compressed by increasing 

 the pressure on the lid or piston. If, now, the gas consists 

 of a large number of moving particles, and the pressure 

 exerted by it on the walls of the vessel arises from the 

 impacts of the particles against these walls, then equilibrium 

 results when the resultant action of the impacts on the lid 

 is equal to the pressure applied to it. 



If the gas is compressed and the volume diminished, 

 the number of impacts of the now more closely packed 

 particles against the walls increases, and for two reasons : 

 first, there is a larger number of particles in the layer of gas 

 immediately adjoining the walls ; and, secondly, as the par- 

 ticles are more crowded together, they collide oftener, and, 

 hurled back by the collision, are oftener flung against the 

 walls. If, by the compression of the gas, the volume is 

 diminished in the ratio 1 : s 3 , the distance between any pair 

 of particles is diminished in the ratio 1 : s ; the number of 

 particles, therefore, in the bounding layer, which is in 

 contact with a given area of the walls, is increased in the 

 ratio s 2 : 1 ; further, the number of collisions that take place 

 between the molecules in a given time is increased in the 

 ratio 5:1; and in this same ratio also is the number of 

 impacts of any particle in the bounding layer against the 

 walls increased. Since, then, the number of impinging 

 particles is increased in the ratio s 2 : 1, and the number of 

 impacts by each in the ratio s : 1, the number of impacts 

 against a given part of the walls in a given time is increased 

 in the ratio s 3 : 1, which is the inverse of the ratio in which 

 the volume of the gas is diminished. The pressure, therefore, 

 of a gas varies inversely as its volume. 



Boyle's law is thus deduced from the hypothesis of 

 molecular impacts. 



7. The Admissibility of the Hypothesis 



Since Boyle's law can be deduced also from quite dif- 

 ferent assumptions, this first consequence of the theory is 



