Mr Knightley has come over for a chat - he's got business with Mr Woodhouse, and he wants to talk to Emma. First, he wants to find out how she liked Jane Fairfax - and he's disappointed that she's not happier with Jane, whom he defends as being reserved or diffident (a word he's using in the sense of being slow to talk). Notice how he moves chairs to be closer to Emma? (I sure did!)
Mr Elton is to be married
Of course, Mr Knightley wants to tell Emma a piece of gossip that he's heard in town, only he gets scooped by Miss Bates and Miss Fairfax. Of course, Miss Bates manages to hold two conversations simultaneously - one on the haunch of pork that Emma sent to them, interspersed with information about Mr Elton and inquiries as to how Mr Knightley knew about Elton's pending marriage.
Mr Knightley, who was smart enough to figure out what Emma was up to, cannot help giving a bit of detail to Emma as a way of tweaking her:
"It was short--merely to announce--but cheerful, exulting, of course."--Here was a sly glance at Emma. "He had been so fortunate as to--I forget the precise words--one has no business to remember them. The information was, as you state, that he was going to be married to a Miss Hawkins. By his style, I should imagine it just settled."Everyone in the neighborhood knew Mr Elton had a thing for Emma
It becomes quite clear from Miss Bates's comments that the entire town gossiped about how Mr Elton aspired to marry Emma.
"--A Miss Hawkins!--Well, I had always rather fancied it would be some young lady hereabouts; not that I ever--Mrs Cole once whispered to me--but I immediately said, 'No, Mr Elton is a most worthy young man--but'--In short, I do not think I am particularly quick at those sort of discoveries. I do not pretend to it. What is before me, I see. At the same time, nobody could wonder if Mr Elton should have aspired--Miss Woodhouse lets me chatter on, so good-humouredly. She knows I would not offend for the world."Harriet's news, and her reaction to Mr Elton's marriage
When Harriet comes busting into Hartfield with news, Emma is certain that Harriet has already learned of Mr Elton's plans, but no: Harriet's news is that she ran into Robert Martin in Highbury as she took shelter from the rain inside Ford's (a local emporium). In fact, he came in with his sister, Elizabeth, who was perfectly ready and willing to ignore Harriet, but Robert Martin talks his sister into saying hello to Harriet, then does the same himself - and then runs after her to warn her not to take a certain route to Hartfield because he was sure it was flooded. What a nice man! Emma - you idiot - he's such a great guy, and based on his conduct, so much kinder and better mannered than Mr Elton - and I say that based on Mr Elton's conduct thus far, and without reference to what is to come!
Even Emma realizes that the Martins have acted really well, and that their behaviour is that of people with genuine feelings for Harriet. Of course, Emma talks herself out of pitying them overly much, based on her conviction that they have disappointed hopes and ambitions, because in delusional Emma-land, she thinks the Martin family hoped to elevate themselves by their association with Harriet. (Only Emma would - or does - think that the Martins were "lower" than her friend Harriet on the social hierarchy.)
In the end, Emma cuts Harriet off by springing the Elton news on her - and Harriet initially takes the news far better than Emma had anticipated, because she's still so twitterpated about having run into the Martins.