However with the alcohol system that we use, density changes very little with composition and it is reasonable to assume that WEIGHT HOLDUP in each stage stays CONSTANT.

• 然而就象我们看到酒精挥发体系一样，挥发物的成分和密度等随时间的变化极其缓慢，于是我们可以假定在每一个阶段摩尔百分量是不变的。