Thomas saved Winston when what happened?

In Chapter 16, a trapdoor opens in the tunnel, revealing bright sunlight. As they move to get out of the tunnel, Winston looks at a giant glob of silver liquid coalescing on the ceiling of the tunnel. When it becomes a ball, it defies gravity and flies straight into Winston’s face. The boys watch in horror as they see what the death ball really is. The silver coalesces around Winston’s head, leaving scars and heat wherever it touches. With Thomas’s help, Winston is able to throw the mass of gel off of his head. The ball of silver goop flies down the tunnel. Winston is physically and mentally scarred by the event, but he is alive thanks to Thomas.