We have a table of the elements with properties varying periodically with atomic mass, so we compress all of that information and refer to it as the Periodic Table.

我们有一个元素表，其中的元素的特性,随着原子质量的改变而引起周期性的变化，所以我们将所有的信息精简,将它变成一个周期表。

麻省理工公开课 - 固态化学导论课程节选

So, instead of having the periodic variation of water, or the periodic variation of air density here we're talking about an electric field.

不像水或者空气密度的,周期变化，我们这里,讨论的是电场。

麻省理工公开课 - 化学原理课程节选

So, for example, we have this average level, and then it can go high where we have the peak, or it can go very low. We can also discuss sound waves, so again it's just the periodic variation of some property -- in this case we're talking about density, so we have high density areas and low density areas.

例如这是平均位置，在峰的位置水位高，在谷水位低，我们也可以,讨论声波它也是某种,量的周期变化，在这里我们,讨论的是密度,我们有高密度区和低密度区。

麻省理工公开课 - 化学原理课程节选