By the end of the 19th century it was becoming apparent that electricity had
an intimate relationship with matter.
With **by + a point in time**, the tense depends on whether you're talking
about the **past**, **present**, or **future**.
### 1. By + past time → usually Past Perfect
> **By 1900, scientists had discovered many new elements.**
>
> 到 1900 年时，科学家已经发现了许多新元素。
The action was completed **before** that past time.