"I see."

Annoying, but reasonable.  She'll double-check the mass, relative to her initial volume measurements, before she signals her readiness, but at this point she is ready.

One advantage of letting each of the students proceed through the steps of their first potion at their own pace is that they generally do end up doing it at wildly different speeds, allowing him to hover supportively for each of them individually when they first turn on their burners. This is useful because every so often someone manages somehow to light themselves on fire doing that, although Ophelia has been so diligently careful so far that he really doubts she'll be among them. 

He'll give her steps-so-far a once-over, checking that nothing seems to be amiss, and then nod supportively.

Her burner flares to the precise temperature indicated, because she will be very cross with it if it does not.

"I believe I am ready to continue, Professor."

"Well done, Miss Prince. Yes, you're doing very well so far, go right ahead."

The next thing she is supposed to do is monitor the cauldron over its brewing period as it comes to temperature, and turn it off pronto if it does any of the following list of things this potion shouldn't do (emit smoke, turn a color other than this nice shade of lavender, make high pitched shrieking noises, etcetera).

And so she shall do.

Ophelia has been meticulous in her work and so her potion will not do anything it is not supposed to do, as it heats. She may notice, however, a behavior that is unremarkable to wizards but quite unusual to a mundane chemist: as she monitors its temperature, it heats up nonlinearly, as though the perfectly smooth liquid, which is not in any way freezing or vaporizing, has a dozen different melting-like phases.

Around her, meanwhile, various other members of her class will, with various degrees of struggle, perform the same procedure. Many of them are doing it badly, but Slughorn, it transpires, is actually quite good at gently catching people before they do anything dangerous; despite the ponderous speed at which he moves about the classroom, he seems to always be in exactly the right place to cheerfully remind someone that they are about to skip a step, or that they have miscounted, or in one case to smoothly catch, with a levitation charm, a dropped bowl of ground snake fangs before it hits the ground. The kindly and reassuring smiles that Professor Slughorn offers the students who need a lot of this type of help are not, quite, perfectly sincere - he would, perhaps, prefer to spend a larger fraction of his time answering interesting questions - but handholding children through following the directions is, he is aware, a necessary component to getting to have more competent older students, so he is doing his best, and his best is fairly impressive.

Hmm.

She takes notes on the Interesting Data as best she can, and makes concurrent notes of everything that occurs along with it such as changes in color, scent - she carefully wafts the air - oh rates of change is a good one, she's going to time each phase and phase-change as best she can - perhaps texture, though she is loath to do anything not on her list of known safe procedures such as 'insert any objects into the potion', not when she understands so little - hmm, she might be able to get mass by backforming from volume if she can do anything like 'determine weight of contents of cauldron', it's a shame she doesn't have a proper way to measure that as she goes - and, in fact, is the potion increasing, decreasing, or staying the same volume, as best as she is able to determine?  Does it look to be steaming anything off?  Does this correlate with anything else she is actually able to determine at this time, if so?

There is a bit of attempted color-component-divining she tries with a carefully-controlled Lumos Spectra.

...Really, she wants to do this again, but properly.  Like an experiment, rather than a recipe.

...It seems she may have a chance to get some data even now, though - one of her neighboring table's cauldrons seems to be going - too fast, is all she can say about it with any confidence.

"...Excuse me.  I think you might have set your fire too hot - I couldn't help but notice that, judging by the rate of progress of my own potion, yours is going through the various phase-shifts that I've so far identified rather quickly.  ...I think it should be possible to recover from this - maybe by drawing out the stage four phase shift, it's in the right, direction, and if the rate of change is independent of the wattage - rate of energy transfer, I mean - I think it'll get back on track, somewhat - but if that's something you want to try, we should ask Professor Slughorn about it."

And so it goes, for weeks, and then months. 

Ophelia Prince studies relentlessly, does everything meticulously perfectly first try even if it takes her five times as long to try, collects knowledge and strength and resources, asks completely unhinged questions in class, and emits rumours nearly as fast as she collects them. (Five years from now, they will say about her, like it is an insult, knew more curses in first year than most of the seventh years, and this is not, precisely, untrue, though it will be some time before she has the channeling capacity for even a quarter of them.)  

Ophelia Prince is a marvel - a prodigy - a thing that has not been seen in the House of Slytherin in generations - 

- and she is, equally, for all the same reasons, a threat. 

She survives a dozen would-be assassinations before the winter holidays. Some of them are because Andromeda is habitually carpeting her in invisible defensive charms at breakfast; some of them are because Hogwarts is not completely bereft of adult supervision and if you do stupid assassination attempts you end up in detention instead of actually getting anywhere near your target; and some of them are because Ophelia on her very own merits is inexpressibly paranoid.  Sometimes that last thing makes the difference, no matter how many people are trying to protect you, between dying and surviving, when you get unlucky enough. (None of them are from her roommates. It's not actually clear who they are from, but it's pretty clear Annette and Karina are taking her speech from that first night quite seriously. It's mostly mysteriously sourceless hexes in the hallways.) 

Professor Weasley starts giving her detention every time she says something 'too helpful' to one of her classmates in his class, claiming that they ought to learn it themselves. This mainly has the very intentional and extremely funny result that the other first-year Slytherins start trying to plausibly deniably help each other with the Defense material so that they can feel very smug about getting one over on him, and also makes them less neurotic than the rest of their House about Ophelia, because they're totally "doing better than she is" at this specific thing. The equally intentional secondary effect, of course, is that attending detention is mandatory, unlike going to office hours, which gives him the opportunity to correct anything particularly stupid Narcissa teaches Ophelia. (Narcissa is actually quite good at duelling, most of the things she has to teach are genuinely helpful, but her advice occasionally suffers of "has never actually been in a real fight in her life" syndrome.) 

Ophelia, of course, has been playing into the bit of Septimus' 'detentions' to the hilt, with the faint smile of an inside joke on her face when no-one is looking.  She really does appreciate him.

She's also been honing her Legilimentic senses - some of those sourceless hexes in the corridors are perhaps dodged before they're thrown.  (She has not revealed this capability to Narcissa, because she is not a fool.  Septimus, on the other hand, knows.)

She makes steady progress through the library, and is often seen with Potions supplies - she's taken to brewing a few ubiquitously useful ones, and selling them to all comers, a Practical Potions price sheet pinned to each House's common room board.  A reputation for reliable business will be of use, she imagines.

(Those who get caught trying to assassinate her, get cut off quite firmly.  And those who don't get caught...  There is a way she looks at you, and drops an innocuous comment.  "I've always thought that the potential of a well-placed Jelly-Legs Jinx is quite underrated.  It's quite hard to recover from having one's brain strewn over the floor, I understand.  Not that I've any such designs.  Live and let live, hmm?")

She is a tiny Slytherin delight. He is definitely just saying this because he is a proud teacher and not also because some part of his brain is convinced that Ophelia is roughly what you'd get if he and Cedrella had had a kid (which for war reasons they aren't planning to).

In any event, the tertiary and totally unintended effect of the detention thing is that when Nelya Vector, a seventh-year Ravenclaw, turns up outside of office hours in late March with a NEWT project she wants Professor Weasley's advice on, Ophelia is also there, doing punitive homework (read: taking notes out of a book she is not allowed to leave the room with for legal reasons). Her psychic vibes are 97% academic anxiety, 2% ambient-teenage-crush-on-one-of-her-classmates anxiety, and a tiny 1% undercurrent of paranoia that one of her academic rivals might have sabotaged her thesis project in some way that she has been unable to detect, which would be SO embarrassing. 

"Hi, Professor," says Vector, "sorry, I know it's not office hours, um, it's just that Spellman wants an update on this project first thing tomorrow morning and I thought I could fix it but I cannot get the stupid thing to harmonize even though all my math says it should?" 

Weasley blinks at her. " ... happy to help if I can, Miss Vector, but I'm not really a Runes guy, you might want to try Flitwick?"

"He's not in his office right now and anyway I think it's the shield charm that's the problem so I thought maybe that's kind of your area?" 

"Huh. Fair enough, sure, let me take a look." 

When he touches the paper, it explodes. 

What is Ophelia's first-couple-seconds-or-so response to this occurrence? 

Wizards are resilient, this she knows.  But as the blast begins to spark, she has a feeling that if she does not stop this spell right now these particular wizards will not be resilient enough.

Her ebony wand trails the sparks of Vermillious as she whips it through "Protego!", lightning-quick, shaping the shield like a cauldron to point the blast-force away - while the acacia-vinewood wand leaps to her other hand, now free of pencils, for a very precisely directed Finite Incantatem.

Nelya shrieks and backpedals. Her instinct is also a shield charm, a more advanced protego which it will very shortly be clear is the vector (pun intended) through which she was sabotaged; unlike Ophelia's, it does not go the direction she wanted it to, but rather erupts in her face and flings her unceremoniously into a wall, which on the bright side is bludgeoning damage, against which wizards indeed are quite resilient, and on the less bright side still kind of sucks immensely. 

Meanwhile, Sept's priorities are putting himself physically between the explosion and the children (especially Nelya, who until a half-second ago was much closer to it), trying to stabilize it before it gets big enough to go around any shield he can conjure this close to it that won't also include it, and actually defending himself from it, in that order. The first thing is rather easier with the blastwave abruptly less circular, and the second thing makes him cackle in adrenaline-fueled delight when the finite whistles past his head at about the time he's finished casting a diagnostic. Rather than waste precious milliseconds casting another spell himself, he just reaches out and pours about six Ophelias worth of raw power into her already meticulously constructed spellform. 

The explosion gets to about an inch past him before it disappears into the finite like an object abruptly discovering gravity. 

"Ow," he says, faintly, disgruntledly, and then hits the ground. 

...Right.  Does she have potions for this.  She should have some things...

"Standard antiinflammatory painkiller," she doesn't actually throw it at Nelya but she's moving very fast, "you might be concussed though, with that hit to the head, so we definitely need a proper medic," and someone to untangle whatever else was in her head, "I'm not really past cuts and bruises yet -" she gets to Septimus next, annnd he's passed out, "damn it, I can't cast Rennervate yet and you shouldn't be casting after that backlash, maybe smelling salts -"

Meanwhile, the castle's ravens are warking in alarm; one flutters through the door of Septimus' office, and there's already another attempting to get the attention of the castle's healer in the infirmary.

-- She can work with this -- "Find the Healer, give them this note, I'll give you something nice later okay --"

In Defense office.  Runes proj sabotaged explosively, dispelled; Sept. KOed, student thrown at wall, likely concussed, suspect victim of mind magic.

Wark (affirmative)!

Poppy Pomfrey (age twenty-five) has been the Hogwarts nurse on her own recognizance for approximately eighteen months. (Before that she was the assistant nurse, and before that she worked at St. Mungo's, which the usual recommendation to aspiring Healers is that you should do for at least five years before you attempt to be solely responsible for anything, ever.) She has mostly been responsible for what she'd consider minor issues. A Muggle doctor would boggle, of course, at the rate at which Hogwarts students, despite being about as easy to injure as an average grizzly bear, manage to break various bones, lose alarming amounts of critical bodily fluids, imbibe lethal poisons, &c &c., but this is normal.

A teacher getting injured is not normal, though in fairness it sure does happen to the Defense professors in particular at an impressively elevated rate to the baseline. 

"Thank you," she says to the raven, honestly barely noticing that it isn't an owl, and hurries off at once. For reasons not unrelated to the aforementioned elevated rate of Defense professors having problems, the Defense office is not very far from the hospital wing. 

In the meantime, Nelya stares at the baby Slytherin who just handed her a handmade potion with an expression of utter bewilderment for about fifteen seconds and then recognizes her face. "..... definitely thought 'Tima was exaggerating about your, uh," she gestures vaguely at Ophelia's general everything, with a faint air of bemused apology. "If you want I can cast a perfectly fiiiiinope, no, nevermind, I just blew myself into a wall with a shield charm, I'm cursed or something, yep, healer, good, smart." She contemplates the homemade painkiller, decides it's probably not worse than the concussion even if it's bad, chugs it, and then leans dizzily back against the wall. "Ugh. I am going to be so glad to get out of this fucking - sorry - this stupid place. Things keep happening." 

Pomfrey at this point arrives, diagnoses the scene as containing one (1) fully lucid human person, and says to Ophelia, "What's this about mind magic?" 

"It's quite alright.  This is...  Above average happeningness.  I will not permit it to continue."

And then the healer arrives.

"I don't expect that Miss Vector miscast a shielding charm so badly as to blow herself into a wall, by accident, ma'am.  Which leaves mind magic of some sort as a likely culprit, though oddly, given the likeliest suspects are from Slytherin's upper years, this doesn't look like the Imperius...  Not unless it's been done far more subtly than anything we've ever known.  ...Oh, hm, there is the Confundus.  That would do it, I suppose.  And one would hope - well, no, one would not normally hope, but, you know - that someone plotting an assassination of at least two people by 'spell mishap' would be more subtle than to use the Imperius about it.  From what little I've heard, it's a painfully blunt instrument.

"Medically speaking, I've given her the Hogwarts standard anti-inflammatory painkiller potion as hopefully a prophylactic for potential brain swelling et cetera.  ...I think that's a thing."