One of the themes in Chapter 7 is the increasing heat. This symbolizes the passion between Gatsby and Daisy as well as the increasing tension between Daisy, Tom, and Gatsby, and later between George and Myrtle. Leaving the house, they stop at Wilson's garage. George Wilson claims he will be taking Myrtle away somewhere because he has grown suspicious that she's been having an affair. Myrtle watches their discussion from an upstairs window and believes that Jordan (who'd been riding in the car with Tom) is actually Tom's wife. Deception and miscommunication are prominent themes in this chapter. Tom is having an affair with Myrtle. Gatsby has an affair with Daisy. The tension rises as these lies slowly become uncovered. Myrtle is killed by Daisy's reckless driving. This signifies everyone's reckless behavior. All of the lying and deception has unraveled and ends with a fatal consequence.
In Chapter 8, the characters are reeling from the destruction they've caused. This applies to Myrtle's death and the destruction in their own lives and relationships. Gatsby is still clinging to some hope of a future with Daisy. With all of the deception and corruption among this group of people, Nick still sees something innocent and idealistic in Gatsby. He leaves him saying that he is "worth the whole damn bunch put together."
Wilson, grieving in the aptly named Valley of Ashes, plots his revenge. With Gatsby dead among the leaves floating in the pool, the lasting image suggests wasted opportunity in life. Things have fallen (the leaves) apart. Lives have been wasted because corrupted means had been used to achieve certain dreams. If Chapter 7's main themes are deception tension, and destruction, then Chapter 8's main themes are mourning and regret.