What is the "intoxicating draught" Victor refers to, and why would he question if Walton has drunk it?

Letter IV mostly
narrates the early part of the friendship betweenand.  As Victor recovers from his brush with
death on the ice, he and Walton have a great deal of time to talk, and they find that they get
along very well and begin to really care for one another as brothers.  

Walton takes this opportunity to open up to Victor about his goals for this dangerous
expedition to the Arctic.  He longs to find the Northwest Passage, a waterway that connects the
northern Atlantic Ocean to the Pacific Ocean and to understand the mystery of how the compass
works.  Discovery, in general, is Walton's primary goal, and he yearns to make a significant
contribution to humanity's knowledge and thereby confer an "inestimable benefit" on
"all mankind to the last generation" (Letter 1).  

As
he...