What he felt wasn't pain, exactly, but something very much like it, a sensitivity that made light and noise intolerable, as if the exposed blade of the world had been thrust into his skull.

Isaac understood his own specialness. He knew he had been created in an attempt to communicate with the Hypothetical, and he knew he had been a disappointment to the adults around him. He knew other things, too. He knew the vacuum of space wasn't empty: it was populated by ghost particles that existed too briefly to interact with the world of tangible things; but the Hypothetical could manipulate these ephemeral particles and use them to send and receive information. The Martian technology embedded in Isaac had attuned his nervous system to this kind of signaling. But it never resolved into anything like the comfortable linearity of words. Most of the time it was a sense of distant, inexpressible urgency.

Sometimes—now—it was more like pain. And the pain was connected with the approaching cloud of dust and ash: the unseen world heaved with an invisible tumult, and Isaac's mind and body vibrated in concert.

He was aware, too, of being lifted into the rear seat of the car, of being strapped in by hands not his own, of the voices and concerns of his old and new friends. They were afraid for him. And they were afraid for themselves. He was aware of Dr. Dvali ordering everyone into the car, the slamming doors, the revving engine. And he was glad it was not Dr. Dvali who held his head and soothed him (it was Mrs. Rebka), because he had come to dislike Dr. Dvali, almost to hate Dr. Dvali, for reasons he didn't understand.

* * * * *

Mrs. Rebka wasn't a physician but she had trained herself in basic medicine, as had the other Fourths, and Lise watched as she administered a sedative, pricking the boy's arm with an old-fashioned syringe.

Isaac began to breathe more deeply and his screaming eventually ebbed to a sigh.

They drove. The vehicle's headlights cut columns of light into the falling dust. Turk was doing the driving on behalf of the Fourths, trying to get out of the foothills before the roads became impassable. Lise had asked whether they shouldn't take Isaac to a hospital, but Mrs. Rebka shook her head: "There's nothing a hospital can do for him. Nothing we can't do for him ourselves."

Diane Dupree watched the boy with wide, anxious eyes. Sulean Moi also watched him, but her expression was more inscrutable—some combination, it seemed to Lise, of resignation and terror.

But it was Mrs. Rebka who allowed Isaac to rest his head on her shoulder, who reassured him with a word or the silent pressure of her hand when the bounce and rattle of the car disturbed him. She smoothed his hair and dabbed his forehead with a damp cloth. Before long the sedative put him to sleep.