费恩曼物理学讲义第一卷与第二卷的区别

费恩曼物理学讲义中的问题.
我用中文说可能翻译的不太确切,直接上原文了.now let us put twice as many molecules in this ***,so as to double the density,and let them have the same speed,i.e.,the same temperature.then,to a close approximation,the number of collisions will be doubled,and since each will be just as"energetic"as before,the pressure is proportional to the density.if we consider the true nature of the forces between the atoms,we would exoect a slight decrease in pressure because of the attraction between the atoms,and a slight increase because of the finite volume,they occupy.如何解释文中的因为原子之间的吸引力而导致压强轻微减小?