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“Now, don’t worry about it, son,” he said hurriedly; “We’ll see to it that you aren’t punished for it. It’s all right. We just want to ask you a few questions.”

“Sure, Doc; anything,” said MacNeil. But he still looked apprehensive.

“Have you been dosing yourself pretty regularly with these things?”

“Well … uh … well, yeah. Sometimes.” He smiled feebly. “Sometimes I didn’t feel so good, and I didn’t want to bother the medics. You know how it is.”

“Very considerate, I’m sure,” said Smathers with just the barest trace of sarcasm, which, fortunately, fell unheeded on MacNeil’s ears. “But which ones did you take every day?”

“Just the vitamins.” He paused. “And … uh … maybe an aspirin. The only things I took real regular were the vitamins, though. That’s all right ain’t it? Ain’t vitamins food?”

“Sure, son, sure. What did you take yesterday morning, before you got so sick?”

“Just the vitamins,” MacNeil said stoutly. “I figured that since you docs was takin’ care of me, I didn’t need no medicine.”

Dr. Smathers glanced up hopelessly at the other two men. “That eliminates the vitamins,” he said, sotto voce. He looked back at the patient. “No aspirin? No APC’s? You didn’t have a headache at all?”

MacNeil shook his head firmly. “I don’t get headaches much.” Again he essayed a feeble smile. “I ain’t like you guys, I don’t overwork my brains.”

“I’m sure you don’t,” said Smathers. Then his eyes gleamed. “You have quite a bit of stomach trouble, eh? Your digestion bad?”

“Yeah. You know; I told you about it. I get heartburn and acid stomach pretty often. And constipation.”

“What do you take for that?”

“Oh, different things. Sometimes a soda pill, sometimes milk of magnesia, different things.”

Smathers looked disappointed, but before he could say anything, Dr. Petrelli’s awed but excited voice came from behind him. “Do you take Epsom salts?”

“Yeah.”

“I wonder–” said Petrelli softly.

And then he left for the lab at a dead run.

* * * * *

Colonel Fennister and Major Grodski sat at the table in the lab, munching on banana-pears, blissfully enjoying the sweet flavor and the feeling of fullness they were imparting to their stomachs.

“MacNeil can’t stay in the service, of course,” said Fennister. “That is, not in any space-going outfit. We’ll find an Earthside job for him, though. Maybe even give him a medal. You sure these things won’t hurt us?”

Dr. Pilar started to speak, but Petrelli cut him off.

“Positive,” said the chemist. “After we worked it out, it was pretty simple. The ‘poison’ was a chelating agent, that’s all. You saw the test run I did for you.”

The colonel nodded. He’d watched the little chemist add an iron salt to some of the fruit juice and seen it turn red. Then he’d seen it turn pale yellow when a magnesium salt was added. “But what’s a chelating agent?” he asked.

“There are certain organic compounds,” Dr. Petrelli explained, “that are … well, to put it simply, they’re attracted by certain ions. Some are attracted by one ion, some by another. The chelating molecules cluster around the ion and take it out of circulation, so to speak; they neutralize it, in a way.

“Look, suppose you had a dangerous criminal on the loose, and didn’t have any way to kill him. If you kept him surrounded by policemen all the time, he couldn’t do anything. See?”

The Space Service Officers nodded their understanding.

“We call that ‘sequestering’ the ion,” the chemist continued. “It’s used quite frequently in medicine, as Dr. Smathers will tell you. For instance, beryllium ions in the body can be deadly; beryllium poisoning is nasty stuff. But if the patient is treated with the proper chelating agent, the ions are surrounded and don’t do any more damage. They’re still there, but now they’re harmless, you see.”

“Well, then,” said the colonel, “just what did this stuff in the fruit do?”

“It sequestered the iron ions in the body. They couldn’t do their job. The body had to quit making hemoglobin, because hemoglobin needs iron. So, since there was no hemoglobin in the bloodstream, the patient developed sudden pernicious anemia and died of oxygen starvation.”

Colonel Fennister looked suddenly at Dr. Smathers. “I thought you said the blood looked normal.”

“It did,” said the physician. “The colorimeter showed extra hemoglobin, in fact. But the chelating agent in the fruit turns red when it’s connected up with iron–in fact, it’s even redder than blood hemoglobin. And the molecules containing the sequestered iron tend to stick to the outside of the red blood cells, which threw the whole test off.”

“As I understand it, then,” said Major Grodski, “the antidote for the … uh … chelating agent is magnesium?”

“That’s right,” said Dr. Petrelli, nodding. “The stuff prefers magnesium ions to ferrous ions. They fit better within the chelating ring. Any source of magnesium will do, so long as there’s plenty of it. MacNeil was using milk of magnesia, which is the hydroxide, for ‘gastric acidity’. It’s changed to chloride in the stomach. And he was using Epsom salts–the sulfate, and magnesium citrate as laxatives. He was well protected with magnesium ions.”

“We tried it ourselves first, naturally,” said Dr. Pilar. “We haven’t had any ill effects for two days, so I think we’ll be able to make it until the ship comes.”

Major Grodski sighed. “Well, if not, I’ll at least die with a full stomach.” He reached for another banana-pear, then looked over at Petrelli. “Pass the salt, please.”

Silently and solemnly, the chemist handed him the Epsom salts.