She observed, “That many people don’t just vanish into thin air.”

“Nor did these,” he agreed. “They’ve been glimpsed, and there are signs of their passing, but the signs are . . .” He retreated to his original description, finding none other that suited the gravity of the situation. “Terrible. They wander, driven by the weather or whatever boundaries they encounter, bouncing from place to place, and . . . and . . . it is like a herd of starving goats, everywhere they go! They leave nothing behind—they consume all food, all plants and crops, all animals . . . and possibly . . . all the people they meet!”

“People!” Mercy gasped for dramatic effect, and squeezed one of her biscuits until it fragmented in her hand. She let its crumbs fall to the plate, and left them unattended.

“Yes, people! The few who have escaped tell such stories. The missing soldiers and settlers have taken on an awful appearance, thin and hungry. Their skin has turned gray, and they no longer speak except to groan or scream. They pay no attention to their clothing, or their bodies; and some of them bear signs of violent injuries. But these wounded men—and women: as I said, there are settlers among them—they do not fall down or die, though they look like they are dead. Now, tell me, Nurse Lynch, do you know of any poison or illness that can cause such a thing?”

Her instinct was to blurt, Yes! but she gave it half a minute of measured consideration while she nibbled one of the intact biscuits. After all, Ranger Korman hadn’t taken her seriously, and she didn’t know these men half so well. Finally, she said, “Well, I’ve known of men poisoned by putrid foods, canned goods and the like, from battlefield stores. Sometimes those men go a bit senseless. But this sounds to me more like like sap-​poisoning.”

Inspector Galeano asked, “Sap-​poisoning?” and Captain MacGruder looked like he was next in line with questions.

“There’s this drug that the boys use out on the front. Gotten real popular in the last three or four years. When the addicts came into my old hospital, we called ’em ‘wheezers’ because they breathed all funny. And those fellas who use too much of it . . . they go crazy. I never saw any as crazy as what you’re talking about, but I’ve seen close.” Memphis. The Salvation Army. Irvin, who bites.

Captain MacGruder said, “I’ve seen a few sap-​heads in my time, but never as bad as that.” He tapped his fingers on the edge of the table. “They make it out of a gas, you know.”

“I didn’t know.”

“Nasty yellow stuff. They get it from somewhere out west—I’m not sure where, but someplace so far west, they’ve got volcanoes. That’s all I know. They bring it in by dirigible. Pirates run the whole operation, I think. Can’t think of anyone else nutty enough to tangle with it.”

The inspectors sat upright with a snap. “Really?” said Inspector Galeano. “You must tell us more! Señora Lynch, you said you’d seen it make men loco?”

She hesitated, but they looked at her with such an eager air of expectation that she had to say something. “You have to understand, this was a long way from West Texas. And Mexico, for that matter.”

“That’s fine,” Inspector Portilla insisted. “Go on, por favor.”

Mercy spoke not a word of Spanish, but she knew a “please” when she heard it, so she told them the truth. “There was a mission, a place for veterans there. And upstairs were men who’d been separated out from the rest. They were . . . they were like you said.” She nodded at Inspector Galeano. “Thin, and their skin wasn’t the right color, and they were starting to look like . . . like corpses.” The rest came out in a burst. “And one of them tried to bite my hand. I thought he was only trying to lash out at me, ’cause he was mad that I was poking him and prodding him, but . . . no.” She shook her head side to side with fervor. “He wasn’t trying to eat me, or anything. He was just—”

“Trying to chew on your flesh? Señora,” Inspector Portilla pleaded. Then he turned to Captain MacGruder. “You said this was made from gas? Flown in by dirigibles?”

“That’s my understanding,” he replied.

“Then perhaps we can solve two mysteries at once!” the inspector exclaimed. Then he dropped his voice and told them, “A large unregistered dirigible crashed out in West Texas, right around the same time—and the same place—that our forces first disappeared. We believe it originated on the northwest coast, but we can’t be certain.”

Mercy gasped. “You don’t think—”

He went on, “I don’t know what to think. But what if this airship was carrying sap?”

The captain presented another possibility. “Or a load of gas to be processed into sap.”

Everyone fell silent, astonished by the prospect of it—and, frankly, not believing it. Mercy said slowly, “Surely . . . surely if it’s just gas, it would just . . . go away? Rise up into the air? Or maybe blow up, like hydrogen does.”

Captain MacGruder agreed. “Surely it wouldn’t be concentrated enough to . . . to . . . contaminate all those people.”

Inspector Portilla sighed. “You are probably right. But still, it is something to think about,” he told them. Then he excused himself from the table, and his fellow inspector left as well.

Left alone with the Union men, Mercy said, “Damn, I hope that’s not right. I can’t imagine it’s right. Can you? You’ve been on the fronts, haven’t you? Have you seen the men who lie around and look like corpses?”

“I’ve seen sap-​heads, but nothing as bad as what they’re describing—or what you described, either. I don’t like to put it this way, but men who dull their senses with drugs or drink or anything else . . . they don’t live too long on a battlefield. But I’ve seen the glassy eyes, and the skin that starts to look like it’s drying out and going a funny color. Don’t hate me for saying so, but men like that are virtually no good to me, not out on the field. If they make themselves into cannon fodder, that’s probably the best use to be made of ’em.”

“Oh, I understand,” she said. “You’ve got a job to do out there.”

“Yes ma’am,” he said. He might’ve been on the verge of saying more, but the caboose door opened and Malverne Purdue entered with a disgusted look that blossomed into a fake smile. “Men. Mrs. Lynch. So good to find you here.”

Morris Comstock said it first. “Actually, we was just leaving. Sorry. Have yourself a fine supper, though,” he added. Then he pulled himself up out of the chair and followed the captain back through the same door, holding it open for Mercy, in case she wanted to follow.

She said, “Thanks, but I’ll be along in a bit. I might ask for another cup of tea, something to settle my stomach.”

The captain nodded as if to say, Suit yourself. The door smacked shut behind him.

Mercy finished the last few sips of her now-​tepid tea and went for a refresher. When she returned to her seat, she found that the scientist had taken the captain’s spot, and he obviously expected her to join him. She smiled tightly.

“Mr. Purdue,” she greeted him.

“Mrs. Lynch. Nice to see you, of course.”

“Likewise, I’m sure.”

He withdrew a flask and poured some of its contents into his coffee. Mercy thought it smelled like whiskey, but that wasn’t something she cared about, so she didn’t remark it. He said, “Those foreigners who just left the car before I came—I don’t suppose you had a chance to talk to them, did you?”

“A little bit,” she confirmed. “They were just in here, sitting with Captain MacGruder and Mr. Comstock. They invited me to join them, so I did.”

“How very civilized of you,” he said. Some nasty sentiment seemed to underlie the statement, but his sharp-​featured face remained composed in a very portrait of politeness. “If you don’t mind my asking, what was the topic of conversation? I find it difficult to believe that such a diverse group could find much to talk about. Except, perhaps, a mutual dislike of Republicans.”

Because it wasn’t a secret (Lord knew, it’d made enough newspapers), she said, “We were talking about those missing Mexicans out in Texas. That legion that up and disappeared a few months ago.”

“Ah, I see. A relatively safe topic, that.”

“What makes you say so?” she asked.

He shrugged. “Politics are funny,” he said. “But since that Texian is back in his own seat, I guess it gave the rest of the lads something to bond over, since none of them want him on board. It’s a shell game, really. Or, it’s like the old logic puzzles, about how to cross a river with a lion, a goat, an elephant, and . . . oh, I don’t know. Some other assortment of animals that may or may not want to eat one another.” Malverne Purdue took a teaspoon, swirled his mixture of coffee and alcohol, then brought the cup to his lips and took a draft too big to be called a sip.

“I don’t follow you,” Mercy replied.

He gestured with the teaspoon as he spoke. “It’s like this: On board this train we have a great contingent of Union soldiers,” he said, tripping over the word soldiers as if he would’ve liked to say something less complimentary. “We also have at least one Texian, a pair of Mexicans, and probably a southern sympathizer or two someplace.”

“Sympathizers?” she said. “I’m sure I haven’t spotted any.”

“You been on the lookout?” he asked. When she didn’t answer, he went on. “We might as well assume it, ever since St. Louis. Can’t count on anyone in that bloodied-​up territory. Bushwhackers, jaywalkers . . . I wouldn’t trust any of them as far as I could throw the Dreadnought. If there’s not a spy or two somewhere on board, I’ll eat my hat.”

“That’s a threat I’m bound to remember.”