Title: Chemical Symmetry

Rating: K+

Summary: Following their chat about chemistry, the Doctor (10) decides that a demonstration is in order.

Disclaimer: I still don't own them.

A/N: Okay, I couldn't help it, I had to write a follow up to 'Subtle Chemistry.' Which you can find here:

And thank you, thank you, thank you to my beta, saganamidreams, who convinced me to post this, and who originally planted the idea for 'Subtle Chemistry,' and who came up with the title for this one. She's doing wonderfully, can't believe she hasn't just decided to completely ignore me yet. And, thank you to all my reviewers, the response was overwhelming.

I've spent my last few lectures/lab classes/synthesis sessions mulling over whether or not/how to continue and then it hit me. Chemiluminescence, or some variation. This is the result. Pretty much all of what I write is actually realistic within the realm of chemistry. What isn't real, will be as science advances. Who knows, in a few years I might be pioneering the stuff. You probably have no idea what I'm talking about, so I will just stop crapping on and let you read the story.

Chemical Symmetry

"What are you doing?"

A slight smile crept onto his features as Rose finally moved through the doorway and into the lab. Thanks to the TARDIS's tracking abilities he knew perfectly well that she'd been hanging around the open doorway far more than she had any reason to, ever since their little conversation about chemistry. Now, she was taking the plunge and walking into his domain.

All as was planned.

He turned around in his chair with a little more flourish than was needed and crossed his arms over his chest. "Hi."

They paused as they both considered the situation. Perched comfortably on the edge of a tall chair at one of the benches he leaned back to look at her, opting to ignore her question as he so often did and just watch. Only problem with that plan, he knew, was that it gave her exactly the same chance. So while he got to see the questioning eyes, the slightly tilted head and the flicker of her vision around the room, she got to see him.

Looking the part, complete with white lab coat and safety glasses (though resting on the top of his head they probably weren't really that useful), his eyes were dancing with a secret as his lips twitched upwards and his fingers tapped against his arm.

Three seconds passed before she could no longer stand it and asked the question she always used to force the conversation. "So what's our next stop to be?"

"I was thinking the Palladium Two Tetraaqua System…But I didn't bring you in here to talk about that."

She smiled, sickly sweet, to cover the dismay of knowing he was right. "You didn't bring me in here. I came here by myself."

"No," he replied resolutely and swung back around to face away from her, pulling his chair along the bench until he reached the end. "I lured you in here, psychologically."

"Ah." She let herself grin, knowing he couldn't see her; God, his Scottish accent really was beginning to get to her, the way he dragged words out and the foreign inflections. She had to stop thinking like that though, especially now, while he was in the room. "So it's not chemistry today, it's psychology instead?"

Grabbing what he needed he rolled back to the microscope. Getting up after a few seconds of fiddling he stepped back from the chair and offered her a hand. "Take a look."

Her brow creased. Suddenly she'd gone from feeling normal to feeling the familiar tingle that had preceded the previous day's encounter, and yesterday it had just been words. She was on his turf now; who knew what he could do. So, despite the fact that Rose knew perfectly well that this could all end with logic being discarded and everything that she refused to think about happening, she took his hand and let him guide her onto the chair. She leaned forward and looked through the scope.

It was fair to say that with the buildup, the sense of him standing just behind her and the fact she already knew that whatever it was, it would blow her mind, she was fairly disappointed. It was a transparent worm with a reddish streak down the centre. Drawing back, she realized she couldn't even see it on the sample plate. Invisible.

"Pyrocystis fusiformis."

"What?" she asked, pulling back and turning her head to look at him without thinking. Suddenly, she could feel his breath on her face and could see the perfect, unbroken chocolate of his eyes. He was close.

"It's a type of organism." Could she tell that his voice had dropped in pitch and volume? That he seemed to have completely forgotten how to blink and how to breathe?

"Oh." She turned back around and the moment was gone.

Rose could only stare at the little wormy thing with which the Doctor was trying to impress her, because that was what he was doing: trying to impress her. She stopped examining the organism; that exact thought had never hit her so hard. This was the same. This was exactly the same. Both thought they were brilliant beyond anyone's comprehension and were eager to prove it, the old Doctor and the new. A smile passed over her lips before she once again concluded that what she was currently viewing was boring and unimpressive and, ergo, he still had something up his sleeve.

"So?" she queried, swinging around on the chair, careful to give him time to move back. "Where's the impressiveness?"

Yes, all was going to plan.

The smile he sent her, he hoped, was secretive. In fact, she thought it rather magnetic but that was physics and she didn't want to go anywhere near that. He stalked off, moving like he knew what he was doing, like he had plan. What was he up to?

"There." He returned with a clear plastic bag filled with water. At her bemused expression his voice dropped to an anticipatory whisper. "This has taken me all day to organize and it's some of the most amazing chemistry you'll ever witness. Living organisms phosphorescing."

Well, she had no idea what that meant, but this was beginning to sound interesting. Though, there was something off, something that wasn't right in his voice, something that shouldn't have been there or perhaps something that should have been, but wasn't. He didn't sound like he had yesterday.

"Watch this." He transferred the bag steadily to a long hook protruding from the wall, allowing it to hang over the black surface of the bench. He gave the bag a rough shove causing the water to ripple and motioned Rose to come closer so she was watching from above. Momentarily, something flashed blue in the bag, shining against the black background like little blue particles of dust.

"Heh?" he sounded like he expected her to be blown away.

"Wow." She tried to make her voice breathy and leaned forward to try and glimpse the last of the blue light. Was that it? She froze, heart stopping, breath catching and every muscle tensing when he pulled her hair back for her, his hands brushing over her shoulders and neck and gently holding it away from her eyes.

The advantage he gained from her sudden distraction was such that not only did she completely miss his grin, she also forgot her earlier suspicion. He leaned forward, just as he'd planned to. "See? Pyrocystis fusiformis, millions of them. An enzyme, luciferin, reacts with oxygen and releases light."

How she could suddenly be bored was beyond her, but she was. Maybe what had happened the day before was a one off thing, maybe him and his chemistry were never going to do anything for her ever again. And he was so close, his hands still holding her hair, his face probably only a few inches away - she could check if she angled her head - but that would be dangerous. Bored with the chemistry, perhaps, but not so bored as to be able to ignore the Doctor.

She stood, feeling him drop her hair and back away so she had room to get out. "Impressive," she lied. "I better go find us something to eat."

He said nothing until she was about to push open the door. It had worked just as he'd planned. "Rose."

A question? A statement? A request? She turned around to check what it was he wanted and watched, head tilted, as a self-satisfied smirk flew over his face and he once again leaned back against the bench, subtly hitting the switch that turned the lights off.

Everything around her danced with light, light that came from the very air she was breathing. Yellow and red and blue, smaller than dust, invisible as singular items but visible in waves she could see moving with a subtlety only the stillness of air possesses. Her eyes were wide and, after a time, she finally let out a breath only to have it catch in her throat again. The stream of air that had escaped her lungs moved across the room; she watched its path as it slowly dissipated, mingling with everything else.

Laughter bubbled up and escaped as she moved a hand in front of her face, marveling at the light that seemed to play over her skin, specific streams moving in specific directions. Inevitably, she succumbed to the temptation and, throwing her arms out, twirled herself around, stopping after two turns to watch the rest of the room through fast moving streams of color. It was as though the air had become a liquid and she just hadn't noticed.

This was impressive. This was beyond impressive; this was unbelievable, unimaginable, unfathomable and yet real. How had he done it? He. The Doctor. She glanced over at him, catching the end of a tense gaze that flickered away so quickly she wasn't sure it was ever there.

She tried to wipe the sudden fever from her eyes; that he'd done this, on purpose, was having the strangest effect. The otherworldly lights - more otherworldly than any of the other worlds she'd ever visited – were thrilling and enchanting her at the same time and, by association, so was he.

But he couldn't know that.

Did he already? Was that what he'd seen while he watched her? Rose went back to watching the light, her vision continuously dragged back to the Doctor as she found him more of a curiosity than the air.

His breath was shallow, she knew that from the light playing around his face. He was hardly moving, tensed, like she was. The light itself somehow threw his face into an unusual shadow and his white lab coat into a strange tie-dye.

Moving towards him the entire room's air rearranged, everything swirling about before resettling as she stopped, back against the bench beside him. Her hands grasped the edge, her right almost touching his left. "How?"

He grinned. "Subtly." He glanced sideways at her. "Chemically."

Her mouth curved up. "So the plastic bag was just a warm up?"

"Yeah, sort of."

"You tricked me?"

"Yeah. I did." He buried his hands deep in his coat pockets, stepping away but making sure to brush her arm with his on the way. Just out of reach he stopped, standing still, feet shoulder width apart as he admired his work.

"A bit full of yourself aren't you?"

"I know how impressive I am," he replied simply.

Did her really think those exact words? Or did he remember and choose them carefully?

"The chemistry's impressive. Hasn't got much to do with you." Again she was grinning stupidly even though she kept ordering her lips to stop; her tongue slipped out and she noticed the upward twitch of his mouth.

"Is that right?" he pulled a vial from his pocket and took the lid off. "More chemistry then."

"What's that?" she asked, sounding over eager even to her own ears. Yeah, she really was falling for the chemistry.

No answer was provided immediately; he just waited for it to evaporate and enjoyed the show as Rose's face fell completely in synch with the dying lights. To begin with he was unsure it would work but the chemical evaporated beautifully and mixed with the air, rearranging on the molecular level and destroying the light, bit-by-bit throwing the room into total darkness.

"Look." He waited until her eyes left his, allowing him the freedom to watch her reaction, "The molecules are rearranging. For every speck of light, so tiny you'll never distinguish it, there's a metal centre. A single atom of copper. The light comes from the chemicals that surround it, they're excited, producing the energy we see as light. I've released another chemical; it wants to bond to the copper so it fights for dominance, trying to get itself into position to take the copper for its own. The new configuration doesn't produce light so you can't see it happening, the fight for the copper, which is dominant, which is submissive."

"How come the new chemical's so dominant?" Rose asked, voice undeniably husky though she didn't care.

Ah, and now for the punch line, "It's inevitable. It's a better march for the copper. More attraction, a snugger fit, better symmetry." His tone changed suddenly, causing her eyes to refocus and swing around to look at him again. "But since you think the chemistry is so impressive, this will be just as exciting. Just as much chemistry going on here." She could sense the way he must have been smiling, though she couldn't see him since, by this stage, most of the room was pitch black, only a little light creeping in from where she'd left the door open a crack.

He'd won this round: she was still short of breath, eyes wide and her brain no longer compatible with her body, she accepted that but asked herself when the game had become a competition. That was what it was. A competition. It was that thought and only that thought that caused he to do what she did next.

Moving to walk out in defeat, she brushed past him in the darkness, pressing herself against him as much as she dare until she felt him tense, then relax, one of his hands lifting to touch her hip, lingering there until she'd walked out of reach.

The Doctor just stood in the darkness, smiling stupidly at the overwhelming success of his little experiment. He saw her silhouette pause in the doorway to speak once more before disappearing, "I'm loving the chemistry. Call me back when the light show's ready to go again." She hoped that made sense on all the levels it was supposed to.

Switching the lights back on as soon as he was sure Rose was gone, the Doctor sank into his chair and released a breath. This had all started innocently, all been about chemistry, chemistry that was atoms and molecules and electrons. The subtle kind. Now, it was still about chemistry, he just wasn't sure that he was going to be able to keep it from exploding.