Violet doesn't respond. She's too nervous in case she says something completely wrong and freaks him out. She continues to watch his shadow, making sure that she doesn't take too much. As soon as she is done, what has happened to most is either they collapse on the ground from exhaustion or pass out like her friend did. She hopes Jack doesn't do that, but she would be prepared if he did. 

 Five-Sixteenths...Three-Eighths... One-Half! She steps quickly away from him, capping her vial to make sure the shadows doesn't escape. She then makes sure to come closer just in case he falls.