“Since the aim is to reduce the amount of oxygen, another thing you could do is to make the room smaller. To put that in the terms a physicist would use, you need to reduce the room’s volumetric capacity.”
“Using this hole here? How?”
“First, you seal up any gaps around the door, then you introduce some object or objects into the room through this hole. The volume of the room will diminish in proportion to the volume of the object introduced, driving the air out. If you persist, the room volume will eventually diminish to the point that an oxygen deficit can easily occur.”
Yukawa delivered this speech with great solemnity. Kusanagi looked at him and then pointed at the square hole. “And what sort of object can you introduce through this hole? It’s only big enough for a glass bead. Yes, the room is small, but you’d still need tens of thousands — no, hundreds of thousands — of the things.”
“That’s true, if we’re talking about objects that don’t change shape. How about, though, if you used, say, balloons?”
“Balloons. How?”
“You stuff an uninflated balloon through the hole keeping the mouth of the balloon on the outside of the door. You then start to inflate it with air from this side. Once the balloon has expanded to a sufficiently large size, you knot the mouth and let it go. As I explained a minute ago, the volume of the room will shrink in proportion to the cubic volume of the balloons. Provided you use balloons that can be blown up to a large size, it should be a highly efficient method.”
Kusanagi pictured the interior of the room filling up with more and more balloons. How many would you need to fill up fifty square feet?
“That’s certainly a novel idea — but it doesn’t seem altogether realistic.”
“Doesn’t find favor with you? Killing someone by burying them in a welter of colorful balloons strikes me as a surrealistic, humorous, and rather delightful trick.”
“I’ll allow you the surrealistic part. A couple of problems remain, though. First, the victim wouldn’t asphyxiate rapidly, and second, he would come to when he had trouble breathing. Once he realizes that his problem is being trapped in a small room with a whole load of balloons, then all he needs to do is to start popping the things.”
“Perhaps you’re right. Okay, what if the balloons weren’t filled with air?”
“What are you getting at?”
Yukawa smiled meaningfully, then laughed under his breath.
“If you didn’t use air, I suppose you wouldn’t need balloons in the first place.”
23
Detective Sergeant Utsumi opened her almond-shaped eyes wide. “Helium?”
“Yes. You shut the sliding door, lock it, and pump helium gas from a tank in through this little square hole. Helium’s lighter than air, so it stays in the upper part of the room. As more and more helium is pumped in, the air is pushed out of the room through gaps around the door. Hasunuma may be lying on his mattress down on the floor, but the overall concentration of oxygen in the room is still going down. If he realizes something funny’s happening halfway through and gets to his feet, there’s even less oxygen in the upper part of the room than the lower part. He tries desperately to breathe, but now he’s inhaling helium and not air, so he loses consciousness instantly. If his unconscious state persists, he will inevitably lapse into anoxia.” Kusanagi fidgeted with his empty paper coffee cup as he looked up at his team.
They were in one of the meeting rooms at the Kikuno precinct station. Kusanagi had been relaying a hypothetical scenario from Yukawa to Detectives Utsumi, Kishitani, and Muto.
“Vintage Detective Galileo!” said Kishitani with a sigh. “I’d never have thought of anything like that.”
“I discussed it with Forensics. They thought it plausible enough. Sudden loss of consciousness would explain why there were no signs of a struggle in the room or of Hasunuma having flailed around. I also talked to the pathologist who conducted the autopsy. He said that there’s nothing incongruous about helium causing anoxia. In fact, helium would actually help explain why the petechiae is so much less pronounced than it would be with strangulation.”
“In that case, the problem we need to figure out is where the perpetrator got the helium,” Muto said.
“Professor Yukawa had some interesting ideas about that, too. Inspector Muto, you may be familiar with this creature.” Kusanagi pulled up a photo on his phone and showed the display to the three detectives.
“What is it?” Utsumi frowned as she looked at the screen.
“Is it a... frog?” Kishitani tipped his head quizzically.
“That’s what everyone says,” Muto exclaimed. “That’s what I thought when I first saw it.”
“It is the local parade mascot. It’s called Kikunon, apparently,” explained Kusanagi to his subordinates. “It always brings up the rear of the parade. As you can see, it’s an enormous inflatable. Since it’s around thirty feet long, it needs a very large amount of helium gas to inflate it. Obviously, it depends on their size, but Yukawa reckons that you need more than a couple of large high-pressure cylinders to get the job done.”
“You think one of those cylinders might have been used in the murder?”
“I think it’s definitely worth exploring.”
“Fine. I’ll send someone to investigate immediately,” said Kishitani. He turned on his heel and left the room.
“If that’s your opinion, Chief, shouldn’t we be looking elsewhere, too?” Muto asked.
“What do you mean?”
“Kikunon is the only giant inflatable in the parade, but the teams often make use of smaller balloons as props. I didn’t watch this year’s parade, but I imagine it was the same as previous years. Free balloons are also handed out to kids at several locations along the route. They must have had tanks of helium, too.”
“That makes sense...”
A parade is like a traditional Japanese matsuri festival — and no matsuri is complete without balloons.
“If helium was used for the crime,” Muto continued, with a degree of hesitancy, “I think it’s highly probable that the gas cylinder wasn’t stolen.”
“Why do you say that?”
“Helium is actually very easy to buy. When my kids were small, we often got balloons for their birthday parties. My wife would buy helium for them online.”
“Same with a friend of mine,” Utsumi chimed in. “I went to her house and she had all these balloons floating around from her daughter’s birthday party. She’d also bought a little tank of helium to inflate them.”
“Huh.” Kusanagi looked at Utsumi. Given her age, most of her female friends probably already had kids. He kept the thought safely to himself.
“The tanks are disposable. You don’t need to return them after use. They only cost about five thousand yen each,” Muto said.
“Five thousand yen? That’s really cheap.”
“If we assume that this crime wasn’t a spur-of-the-moment thing, then the perpetrator would buy the helium in advance, wouldn’t they?”
“I suppose so. It might be difficult to trace the purchase.” After a moment’s thought, Kusanagi had an idea. “Wait a minute. If you don’t have to return the helium tanks after use, then how would the perpetrator get rid of them?”
“The tanks are bulky and heavy, too. The killer would want to get away from the crime scene as fast as possible; having to lug a gas canister around would really slow him down.” Muto seemed to have grasped the implications of Kusanagi’s remark. He got to his feet. “Let’s round up everyone who’s free and get them to search the vicinity of the crime scene,” he said, and dashed out of the room.