The episode when the real transference begins is in Chapter 2, when Basil unveils his final picture of Dorian and Dorian becomes immensely depressed when he considers his youthful beauty and then contemplates that his own beauty will wither and fade with age. Note what he says to justify his dislike of the painting:
"I am jealous of everything whose beauty does not die. I am jealous of the portrait you have painted of me. Why should it keep what I must lose? Every moment that passes takes something from me, and gives something to it. Oh if it were only the other way! If the picture could change, and I could be always what I am now! Why did you paint it? It will mock me some day - mock me horribly!"
Ironically, of course, the painting will mock, Dorian, though not quite in the way that he imagined. We are not told what forces specifically enact the magic of allowing the painting to age and Dorian to stay in a stasis of youth and beauty, but it is strongly suggested that these forces are demonic. Consider the two main characters in the novel, apart from Dorian, Basil and Lord Henry. These two characters seem to perform the function of the angel and the devil sitting on Dorian's shoulders. Basil always tries to show Dorian what he could be, whereas Lord Henry leads Dorian on into ever-greater depths of hedonism and unthinking selfishness. Certainly this Mephistopheles-like role that Lord Henry plays hints strongly at the demonic.