The following refers to “cub pilot on the mississippi”: brown does not carry out the order delivered by henry because

The simile compares the poison to a fast-moving, toxic element, emphasizing its deadliness.

In this excerpt, we find a simile that intends to explain what the poison was like. The ghost states that the poison Claudius gave him was "swift as quicksilver." Quicksilver is another word to describe the chemical element mercury. Trough this comparison, the ghosts wants to highlight the fact that the poison moved quickly through the body, and that it was extremely toxic.