hers 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 chemi