In Wuthering Heights, when does Isabella write a letter to Nelly and why?

Isabella writes a letter to Nelly after she has eloped with Heathcliff and gone with him to Wuthering Heights.  She is appalled by the conditions she finds there, and horrified at the diabolic change in her new husband.  She writes to Nelly to tell her about her miserable life not four miles away from Thrushcross Grange, and begs her to visit soon.  She also asks two questions of Nelly - first, "how did (Nelly) contrive to preserve the common sympathies of human nature when (she) resided (at Wuthering Heights)", and second, and more chillingly, if "Mr. Heathcliff (is) a man...if so, is he mad?  And if not, is he a devil?"

Isabella had become infatuated with Heathcliff, and had precipitously married him.  Unbeknownst to Isabella, Heathcliff had never loved her, and had married her for her inheritance.  As soon as Isabella marries Heathcliff, he becomes abusive to her.  He brings her to Wuthering Heights that first night, takes her to the kitchen, then disappears, leaving her with his rude servant Joseph, who, after squinting at her "malignantly", ignores her and leaves her to her own devices.  Isabella tries to make friends with "a ruffianly child", Hareton, who stands by the fire, but Hareton responds with an oath and a threat to set the bulldog, Throttler, on her if she does not leave him alone.  Isabella is disgusted to see that the slovenly Joseph stirs the porridge with his dirty hand, and unnerved to notice that all the doors are kept locked in the forbidding dwelling.  Horrified and appalled by the conditions at Wuthering Heights and the depravity of the people living there, and especially of her new husband Heathcliff, Isabella writes to Nelly, begging her to come visit.  Isabella knows that she has condemned herself by committing herself to Heathcliff, and she tells Nelly helplessly, "I do hate him - I am wretched - I have been a fool!" (Chapter 13).