He wasn't quite sure what made him look up, but when he did, it was just in time to pull Ron down as his potion blew up. It had miraculously turned white, and a smell of compost filled the room. It was hot too, and Harry had to keep from crying out in pain. He realised with a start that he had tears on his face and his ears were ringing. Snape's hooked nose swam in his vision as Harry passed out from the sudden vertigo.