It has been a while since I read this book, but I'll give it a shot! "Dash the cup from your lips" means that once Walton understands what has happened, he will not want to drink the "intoxicating draught" anymore and will throw the cup down. This is all metaphorical and the intoxication draught could be many things. It could be the feeling of having power, the glory associated with intellectual conquest, or something having to do with scientific discovery. Either way, the idea is that these men have both been "drunk" by believing in something that ultimately is not what they hoped it would be.