After Dugu Bo handed the Poison Core to Feng Lingtian, Feng Lingtian stayed at the Ice and Fire Yin Yang Well for another seven days.
That night, he drew countless lines on paper, but still could not figure out where that missing trace of poison had gone. The poison set out from the Poison Core, spread along a fixed route, then flowed back. The amounts going out and coming in should have been the same. Yet at the end of every cycle, Feng Lingtian could see the glow dim for an instant at several points along that route. Whatever was missing was there, but he could not see what it was.
At some point, Dugu Bo walked over and stood behind Feng Lingtian.
"My Poison Core was separated from my Martial Soul," Dugu Bo suddenly said. "It was originally part of my Martial Soul and spent decades inside my body. What you're studying now is its circulation outside the body. An external route is external, after all. When it was inside my body, it took a different path."
Feng Lingtian's fingers went still.
Inside the body. The path the Poison Core circulated along inside the body was different from its external route. What was inside the body? Meridians.
He abruptly raised his head. If the Poison Core circulated along the meridians inside the body, would it leave residue where those meridians turned?
He needed a set of meridians whose insides he could see.
On the first day, he carved a wooden dummy, hollowed it out, and used thin bamboo tubes to model the paths of the meridians. The three yin meridians of the hand, the three yang meridians of the foot-he left a recess at the dantian's position.
Feng Lingtian drew a trace of poison from the Poison Core's surface, injected it into the recess at the dantian, and used Soul Power to drive it through the bamboo tubes. The poison set out from the dantian, spread to the arms along the three yin meridians of the hand, then returned to the dantian through the three yang meridians of the foot. One circuit, two circuits, three circuits. Feng Lingtian stared at the inner walls of the bamboo tubes. With every cycle, the poison left extremely minute emerald-green deposits at the bends. One complete cycle left deposits at seven points. The sharper the bend, the more it deposited.
Over the next few days, Feng Lingtian repeatedly adjusted the bamboo tubes' angles, diameters, and paths. Smoothing out the angles reduced the deposits but did not eliminate them; narrowing the diameter made the flow faster, but increased the deposits instead; coating them with herbal juice absorbed some of the deposits, but the poison also corroded the herbal juice itself. No matter what he changed, as long as the poison passed through a bend, it would leave a deposit.
Sometimes Dugu Bo came and stood beside him, watching Feng Lingtian fiddle with the bamboo tubes without a word. Feng Lingtian said nothing either. He simply dismantled each version of the tubes, held them up to the light, and recorded the positions and thicknesses of the deposits. Feng Lingtian knew Dugu Bo was waiting. Waiting for an answer.
At dawn on the seventh day, Feng Lingtian dismantled all the bamboo tubes from the previous six days and reassembled them into a complete meridian circuit. The experiments of those six days had told him which angles produced the least buildup and which paths allowed the poison to flow most smoothly. He combined those optimal choices to put together the seventh version.
He injected the poison. It completed one circuit. Feng Lingtian took the bamboo tubes apart and held them up to the light. The deposits at the bends were smaller than in any previous version, but emerald-green traces remained at all seven bends. He had changed everything he could, yet the deposits still existed.
This was the underlying law itself. When poison flowed through meridians, it would leave deposits wherever it passed through a bend. Not because the angle was poor, not because the flow speed was wrong, but because of the flow itself.
Feng Lingtian laid the seven versions of the bamboo tubes side by side on the ground, his breathing growing faster and faster. All seven versions had deposits at their bends. In particular, the seven deposits in the seventh version were clearly visible, corresponding one for one with the sites of the heaviest buildup in the previous six versions.
"Senior Dugu!" Feng Lingtian's voice carried excitement he could not suppress. "I've found the pattern! Look at these seven versions of the bamboo tubes-I arranged them according to the paths of the human meridians."
Dugu Bo emerged from the stone house, his steps extremely quick. He crouched down and leaned closer to look. He examined them version by version. The first had the thickest deposits, almost blocking half the tube's interior. In the later versions, the deposits gradually grew thinner and fewer. But no matter what changed, traces of the poison's passage remained at the bends of every version. The positions of those deposits were clearly displayed on the seventh version-seven sites.
As Dugu Bo looked at those seven deposit sites, his pupils abruptly contracted.
"These seven sites." Dugu Bo's voice was very low. "They're exactly where it hurts whenever I have an attack."
Feng Lingtian nodded once. "After repeatedly verifying it through these seven versions of the experiment, I reached a conclusion." He looked at the deposits on the bamboo tubes. "When poison flows through meridians, it leaves deposits wherever it passes through a bend. The sharper the bend, the greater the deposit. The meridians inside your body are a hundred times more complex than these bamboo tubes. The buildup can only be worse."
Feng Lingtian paused, then raised his head to look at Dugu Bo. "But if we can make the poison circulate only inside the Poison Core, without passing through the meridians, there won't be any buildup."
"Without passing through the meridians?" Dugu Bo frowned. "I've tried. The Poison Core is an inanimate thing. The poison isn't bound by it. No matter how I compress it, the poison spreads outward."
"You can't contain it because you're compressing it in the wrong direction," Feng Lingtian said. He extended his right hand, and a very faint cluster of cyan Soul Power appeared in his palm. To let Dugu Bo see the principle clearly, Feng Lingtian drew an extremely weak trace of poison from the Poison Core's surface and carefully wrapped it within the center of his Soul Power.
The Soul Power slowly rotated in Feng Lingtian's palm, gradually compressing into a ball the size of a fingernail. Enveloped at its center, the wisp of emerald-green poison slowly turned with the Soul Power's rotation. "You're pressing inward from the outside. The force pressing in from outside and the force of the poison spreading outward resist each other. Resistance produces deposits." Feng Lingtian's palm jolted. The ball of Soul Power stopped spinning, and the poison immediately spread outward, leaving an extremely faint emerald-green residue along its edge.
"But what if we let the Soul Power spin on its own?" Feng Lingtian activated his Soul Power again, drawing it inward layer by layer while driving the center to rotate. The ball of Soul Power spun faster and faster. Wrapped at the center of the waves, the poison churned up and down with the Soul Power's rotation without ever touching the edges.
Dugu Bo stared at the rotating ball of Soul Power, sinking into thought.
"I only just figured it out myself," Feng Lingtian said. "When you originally compressed the Poison Core, you pressed the poison tightly together layer by layer. Afterward, it was still a dead core. But what if you made it rotate while compressing it? Not pressing, but spinning. Compressing and rotating at the same time. Compression gives the Soul Core density; rotation gives it direction. Once it has density and direction, the Soul Core can spin on its own."
Dugu Bo's pupils contracted slightly. "Rotational compression?"
Feng Lingtian nodded once.
Dugu Bo fell silent for a long time.
"How do I do this rotational compression?" Dugu Bo's voice was very low, carrying an impulse he had suppressed for a long time and could barely contain any longer.
Feng Lingtian was silent for a breath. "While compressing, make your Soul Power rotate around your dantian. Compress one layer, complete one rotation. Compress another layer, complete another rotation. Build it up layer by layer, and the instant the Soul Core takes shape, it will begin spinning on its own. Once the Soul Core starts rotating, merge the Poison Core into its center. The Soul Core is alive. The Poison Core will be driven by it, and the poison will rotate along with it inside the Soul Core."
Dugu Bo's breathing quickened a little. He looked at his hands, then raised his head again. The corners of his mouth pulled into a curve, yet a fire shone in his eyes. "I, Dugu Bo, have lived for decades and spent decades compressing it. Now you want me to spin it while I compress it." He stared at Feng Lingtian. "Madman. You little madman."
Feng Lingtian said nothing.
"But I'll go mad with you." Dugu Bo sat cross-legged and closed his eyes. "After decades of compression, it's time to give it a spin."