“What are those machines?” Kusanagi asked Shimaoka.
“Oxygen densitometers. Obviously, we can’t have an observer inside the room, so we’ve set it up so we can monitor the video feed and the densitometer readings from out here.” As he said this, Shimaoka pointed at a folding table that had been erected to one side of the sliding door. On it were two laptops.
The forensic technician who had removed the door handles came back in and said something to Shimaoka. Shimaoka nodded and turned to Kusanagi.
“Everything’s ready. We can start whenever you like.”
Kusanagi looked at Yukawa. Yukawa nodded. “Go ahead,” Kusanagi said to Shimaoka.
Two more technicians came in carrying a cylindrical tank with handles on either side. It was about two feet in height and one foot in diameter with a rubber bulb and special hose on the top. They carefully placed the cylinder on the floor in the middle of the room.
“We need to keep the room well ventilated. Let’s have the front door and the window open,” Yukawa said.
The technicians opened the door and the window as directed. Shimaoka then pulled the sliding door of the small room shut. “Okay, are we ready?”
“Before we start the experiment proper, could you discharge a small amount onto the floor here?” Yukawa said.
“Here in this room?” Shimaoka asked, just to make sure.
“Yes,” replied Yukawa. “I’d like to give Detectives Kusanagi and Utsumi the chance to witness this phenomenon directly.”
“Fine,” said Shimaoka. He nodded to his subordinates.
Leaving the hose dangling onto the floor, the technicians turned several valves and squeezed the rubber bulb on top of the tank until it was flat. A mixture of white vapor and liquid spouted out of the hose and onto the floor.
The liquid disappeared instantaneously, meaning that the floor didn’t get wet.
“What we have here is liquid nitrogen,” Yukawa said. “It has a boiling point of minus one hundred and ninety-six degrees Celsius. Pouring it onto the floor is like dripping waterdrops into a hot frying pan. It vaporizes instantaneously, as you can see. So, what will happen if we take this liquid nitrogen and” — here, he pointed at the sliding door — “we feed it in large quantities into the closed-up small room via the Judas Window?”
“What does happen?” Kusanagi asked.
“That’s what we are going to put to the test right now.”
“Proceed,” Yukawa said to Shimaoka.
Shimaoka gave the word and the forensic technicians got to work. One of them carried the tank up to the sliding door and fed the hose through the square aperture. The other turned on the two laptops. One of the monitors displayed the room’s interior, the other displayed various numeric readouts and graphs.
Yukawa had taken up position behind the technician who was monitoring the computers. Kusanagi and Utsumi followed suit.
“Let’s go,” Shimaoka said.
Just as he had done a minute or two before, the technician squeezed the bulb at the top of the gas cylinder several times. Immediately, a change was visible in the interior of the room on the computer display.
It was filling up with a white mist. The ground sheet, the mattress, and the quilt were only dimly visible through a haze.
“The liquid nitrogen cools the water vapor in the air, condensing it into small gloating droplets of water. You might say we’ve created a cloud inside the room,” Yukawa explained.
“The door cracks... It’s coming through...,” Utsumi murmured.
Kusanagi looked up. Sure enough, white smoke was seeping through whatever gaps there were, though it quickly vanished. When Kusanagi commented on it, Yukawa snapped back, “The temperature’s warm in here, so it’s reverting to water vapor. How’s the concentration of oxygen?” Yukawa asked the technician seated at the table.
“Almost unchanged in the upper part of the room. In the vicinity of the mannequin, it dropped below eighteen percent very rapidly. It’s about to go below seventeen,” the technician replied.
“When the oxygen concentration gets to sixteen percent, you get subjective symptoms like headache and nausea,” Shimaoka said, keeping an eye on the monitors. “Once you go below twelve percent, you start to feel dizzy. And once you go through ten percent, mental functions are impaired.”
Ten minutes later, the oxygen densitometer nearest the dummy was giving a reading of just six percent.
“Six is the level where you experience respiratory arrest. What’s the capacity of the tank?” Yukawa asked Shimaoka.
“Twenty liters. It was almost full when we started. We’ll weigh it when we’ve finished, of course. I don’t think there’ll be much left.”
Yukawa nodded and turned to Kusanagi and Utsumi.
“When liquid nitrogen vaporizes, its volume increases by around seven hundred times. In other words, a twenty-liter tank like this produces fourteen thousand liters. The cubic capacity of that room is around ten thousand liters. Any excess gets pushed out here through the gaps around the door. Since the original air inside and the vaporized nitrogen don’t instantaneously mix, the oxygen concentration is different in different parts of the room. As you can see from this experiment, the oxygen thins out in the lower part of the room first. For anyone asleep inside, there’s a high likelihood of getting oxygen deficiency culminating in respiratory arrest, even if the person were to get to their feet halfway through the process.”
“You’re saying that the murder weapon wasn’t helium after all?” Kusanagi said.
“The helium tank we found was a decoy. It was designed to throw us off track. I owe you an apology there. I was the one to suggest that helium might have been used.”
“Where did you get the idea of liquid nitrogen?”
“I asked myself why the killer had to opt for helium rather than anything else and what he really used for this murder. That’s when it came to me. Perhaps the killer actually wanted the police to think that helium was the murder weapon. Now, if we start looking for a substitute for helium, then what have we got?” Yukawa smiled and pointed at the liquid nitrogen tank. “An inert gas that’s the most abundant element in the atmosphere. And nitrogen is its name. If you use it in liquefied form, all you need is a paltry twenty liters,” said Yukawa, then turned and looked at Utsumi. “I asked Detective Utsumi to check one thing to help me verify my hypothesis.”
“What was that?” Kusanagi asked his female colleague.
“The quantity of moisture in the mattress and quilt when Hasunuma’s body was found,” Utsumi replied.
Kusanagi frowned. “The quantity of moisture?”
“You saw the video feed from inside the room, didn’t you?” said Yukawa. “When liquid nitrogen is pumped into the room, the water vapor in the air turns into a floating white mist. If the temperature in the room goes up, the mist dissolves into the air. If, however, liquid nitrogen keeps on being pumped in, then the temperature in the room does not rise. The room becomes something like the inside of a cloud. It’s an environment where condensation forms very easily. What do you think would happen to a quilt and mattress in a place like that?”
“They would absorb a lot of moisture?”
“When Forensics checked, they found that they were damper than under a normal-usage scenario,” Utsumi said. “In fact, they contained a large amount of moisture; equivalent to about half a cup of excess water.”
“Director Shimaoka,” said Yukawa. “Could we have a look at the interior of the room?”
“Of course. For safety reasons, I’d like you to stand back a little.”
The three of them did as they were told and stepped away from the door. One of the technicians slid the door open but didn’t go in. The oxygen level was still too low.