What were Victor Frankenstein's warnings to Robert Walton in the book Frankenstein?

Once Victor finds out that Walton is willing to sacrifice everything -- even his own life -- in his pursuit of knowledge and discovery, Victor desperately wants to prevent his new friend from making the same mistakes that he did. He says,

"You seek for knowledge and wisdom, as I once did; and I ardently hope that the gratification of your wishes may not be a serpent to sting you, as mine has been." 

Serpents are very often linked to temptation -- an allusion to the serpent in Eden who tempted Eve to eat the apple, the event that led to Adam and Eve's being ejected from paradise -- and so this makes it sound as though Victor believes that knowledge and wisdom can tempt and ruin a person the way the serpent tempted and ruined Eve. He wants to prevent Walton from succumbing to temptation as he has. As Victor relates the story of how he created a monstrous human being, he says,

"Learn from me, if not by my precepts, at least by my example, how dangerous is the acquirement of knowledge, and how much happier that man is who believes his native town to be the world, than he who aspires to become greater than his nature will allow."

Again, he cautions Walton about seeking worldly knowledge, knowledge that is, perhaps, withheld from us for good reason. He believes that Walton will be safer and happier if he does not pursue this kind of knowledge but rather agrees to be satisfied with his home and himself as he is. There is, as Victor knows, danger in attempting to do more than a human being is supposed to do. Further, he tells Walton that

"If the study to which you apply yourself has a tendency to weaken your affections, and to destroy your taste for those simple pleasures in which no alloy can possibly mix, then that study is certainly unlawful, that is to say, not befitting the human mind."

Therefore, because Walton has suggested that he is willing to give up every other thing that brings him joy in life in order to satisfy his one dream of discovery, Victor implores him to understand that this means that his dream is not an appropriate one; it is too all-consuming to be healthy for a human mind. If Walton is willing to sacrifice everything else to it, then it is too dangerous to pursue.