Hartson Brant looked at his son. "You do, don't you, Rick?"
"I know what it is, but I don't know why it's so important to Steve," Rick said. He had read a great deal about heavy water in studying elementary physics. It had many uses in physics experiments.
"Let's see how much you know," Steve directed. "Sound off."
Rick searched his memory, trying to marshal all the facts he knew. "Well," he began, "ordinary water is composed of oxygen and hydrogen. In every water molecule there are two atoms of hydrogen and one of oxygen. The important part, for what we're talking about, are the hydrogen atoms. Hydrogen is the lightest element, and it has the simplest atom. There's just one proton and one electron."
He looked at his father, waiting for a nod to tell him he was on the right track. When the scientist nodded approval, he went on.
"That kind of hydrogen atom has a mass of one, as the scientists say. But there are other kinds of hydrogen atoms, and they are pretty rare, called isotopes. An isotope is just a different variety of the ordinary kind of atom in each element. The thing that makes it different is a change in the nucleus. Well, hydrogen has two isotopes. One kind, which has a mass of two, is found in nature. It is called deuterium. Its nucleus is called a deuteron. Another kind, which can be made in a nuclear reactor, is called tritium. A little of it is found naturally but not enough to count for much."
He took a deep breath. "I hope I know what I'm talking about."
"You're doing fine," Hartson Brant said. "Go on."
"All right. Well, heavy water is made of one atom of oxygen plus two atoms of deuterium, which is the first isotope of hydrogen. In chemistry, there's no difference in the way heavy water acts. You can even drink it. In fact, people do drink it every day, because in ordinary water there is some heavy water. I forget the exact figures, but I think that, by weight, there are five thousand parts of ordinary hydrogen in water and only one part of deuterium."
"That's right." Steve Ames nodded. "Five thousand to one. Now tell us what is peculiar about all isotopes?"
Rick thought furiously and came up with what he hoped was the answer. "I think it's that isotopes aren't as stable as the basic elements. Some are pretty stable, but some are pretty shaky. That's why some of the isotopes of uranium can be split wide open in a chain reaction to make an atomic bomb, and …"
A chill ran through him. His mouth opened. He knew! He knew why heavy water had Steve Ames all excited. He choked:
"Hydrogen bombs!"
Scotty and Barby gasped. Steve Ames and Hartson Brant smiled.
"It's true that one of the possibilities in building a hydrogen bomb concerns deuterium," the scientist said. "But I scarcely think that's the case here. How about it, Steve?"
"Possible, but extremely improbable," Steve agreed. "What I'm most interested in is a use for heavy water Rick hasn't mentioned. Know what a nuclear reactor is, Rick?"
Rick nodded. "It's what the newspapers usually call an 'atomic pile.' We have quite a few in this country, I think. The Atomic Energy Commission said quite a while ago that they used a nuclear reactor with uranium as a fuel to make plutonium, which is the artificial element that can be used in atomic bombs. Besides uranium itself, that is."
"That's right. What I'm interested in is the fact that heavy water can be used as a neutron moderator in a reactor."
Rick looked blank. Steve was talking way over his head. Hartson Brant saw his son's bewilderment and explained: "You've probably heard that the uranium in a reactor is encased in blocks of graphite, which is simply carbon, Rick. It prevents the neutrons from the uranium from simply running wild. Well, heavy water can be used for the same purpose."
"Exactly," Steve said. "So you see, I'm not afraid of the possibility of hydrogen bombs as much as I am of the possibility that somewhere in Asia is a nuclear reactor. Until we get international agreement on atomic weapons, we simply have to keep track of atomic developments everywhere for our own protection. If there's a new country going in for atomic research, and it can build a reactor, it might also be able to build an atomic bomb. Now, don't forget I said heavy water is a legitimate industrial product. We certainly can't object to a nation's manufacturing it. We wouldn't want to. But when it turns up in an odd corner of the world, I think we'd better find out why. If it's a peaceful reason, we'll mark it down and then forget it. If not, we'll make a report to the United Nations."
"Why not report it right now?" Barby asked.
"Good question. The answer is, we're not sure. Remember Carl Bradley was unsure enough to ask for help. If we got up before the UN and started hollering and it turned out to be plain water, we'd look pretty foolish."
"I don't even know how we'd begin," Scotty muttered. "How do you start on a job like this?"
"You'll start by being innocent tourists," Steve said. "You and Rick are students on a holiday, with Zircon, your uncle, as guide and tutor. You'll be interested in a number of things, including hunting. That will give you a good excuse for barging around the country if you have to. But you won't be able to decide what you want to hunt." Steve grinned. "You'll decide after you find out where you have to go. And you'd better learn about Asiatic game animals. For instance, if the trail takes you to Indonesia, you may want to hunt the hairy Sumatran rhinoceros. In the Philippines, you'll hunt timarau, which are a special breed of wild water buffalo. In China, around the coast, you can hunt tigers. In Malaya, if the trail does take you down to Singapore, you can hunt tapir. Same for Siam. In Indo-China you can hunt tigers. Inland in China, toward the Tibetan border, you'd better be hunting bharals."
"That's a wonderful name," Barby said quickly. "What are they?"
"Another name for them is blue sheep," Steve told her. "They're bluish-gray, shading to white in the under parts. The horns are unusual, because they curve outward from the sides of the head, then down and backward."
Hartson Brant paused in the act of filling his pipe and asked curiously, "How do you know so much about Asiatic animals, Steve?"
Steve laughed. "Because I used the same gag once myself." He started for the door. "Talk it over, and think up any questions you can. I won't promise to know the answers, but I'll try. I've got to get Mike started back to Washington to pick up that stuff."
When he had gone, Barby looked enviously at the two boys. "In my next reincarnation," she announced, "I'm going to be a boy. I don't see why I couldn't go, too. A girl would make the group look even less suspicious, wouldn't it?" She scanned the three faces eagerly, then sighed. "All right. I knew it wasn't any use."
"Never mind, towhead," Rick said. He always hated to see Barby's wistful expression when he and Scotty were going somewhere. "Maybe next time."
"Not if next time is another job like this," Hartson Brant disagreed. He studied his pipe stem, his forehead wrinkled thoughtfully. "I'm not quite sure why I didn't object to Rick and Scotty going."
Rick demanded swiftly, "You're not going to object, are you, Dad?"
"No, Rick. If we hadn't been on other expeditions and in some tough spots together, I surely would. But I know you two are able to take care of yourselves. And so is Zircon. Only keep in mind that you may be dealing with an entirely new breed of cats, unscrupulous men who wouldn't hesitate to put you out of the way without a moment's hesitation. So be careful. Be very careful. Don't take risks that aren't essential to your job. And do what Zircon tells you to without hesitation. He's knocked around in some pretty rough corners of the world, and I don't know a man who is better equipped for this kind of job, unless it's Carl Bradley."
The warning sobered Rick even more. Apart from what his father had said, he knew it was also what the information could mean to the security of the country that had prevented the scientist from making a single objection to their going.
"We'll take no risks we don't have to," he promised. "We'll move as if we were walking on eggs, Dad."
And Scotty echoed the promise.
Nothing remained but to wait for Zircon and make definite plans. Steve, who had risen early in order to