翻译：We assumed that there were forces of attraction between molecules which varied rapidly with the distance so that the attraction between molecules that were more than a few ten millionths of a millimeter apart was very small but became considerable when the molecules approached more closely.

【答案解析】：句子的框架是We assumed that there were forces of attraction…。that引导宾语从句。此宾语从句中套嵌一个由which引导的定语从句修饰先行词forces of attraction; so that the attraction between molecules…是so that结构引导的结果状语从句;在此状语从句中，除主干结构the attraction…was very small but became considerable…，又有that were more than a few ten-millionths of a millimeter apart做定语从句修饰先行词molecules和when the molecules approached more closely做状语从句，修饰…but became considerable。considerable译为“相当大（或多的）”。

【译文】我们曾经假定，分子之间存在着引力，这种引力随着分子之间距离的不同而有显著的变化，因而，在间距为千万分之几毫米的分子之间的引力是很小的，但是分子靠得很近时，这种引力会变得相当大。