Harry came to Defense the next afternoon, but found it hard to keep his mind on the discussion. The subject was Grindelwald, the dark wizard whom Dumbledore had defeated so many years before. Professor Takushiki was guiding the class's debate.

"Did Grindelwald start out on the dark side?" she asked, looking around the room.

"No, Professor, but he thought he was meant for greatness and he didn't mind how he got there," Lavender Brown ventured.

"What did he want, then?" she asked. Hands went up around the room.

"Wealth!" "Fame!" "Influence, Miss!" answers rang out.

"And are those bad things? Don't you want them, class?"

"I do - but not if I have to kill someone to get them!" put in Cho, with a musical laugh.

Is that what Voldemort's after, killing the Squibs? thought Harry. It seemed, well, pointless. He remembered the goals he had written for today's assignment: to know about my family, to belong somewhere, to have friends for life... Most people seem to have those already, he mused, is that why those other things seem so important to them?

"So, why did those goals lead Grindelwald into the Dark?" Takushiki was asking.

"He wanted ahh, power, Professor, especially over other people. To, ah, make them do, ah, whatever he wanted." That was Blaise Zabini.

"And to remove his rivals and enemies," put in Millicent Bulstrode.

Crabbe's hand was in the air. "And he never got enough power to satisfy him, Miss." He sneaked a look at Hermione, who flashed him a wide grin.

Does Snape want power? wondered Harry. To be Headmaster, maybe? Or more?

Pansy Parkinson leaned forward in her seat. "And Grindelwald wanted revenge. He killed Meister von Hoffman, his old teacher, because he'd tried to stop some of his experiments years before."

"Yes, we know he held deep grudges. Well done, Bulstrode, Crabbe and Zabini. Two points to Slytherin."

Snape holds grudges, said a voice in Harry's head. Against you, against your father, against your father's friends. Could he have had one against Dumbledore, about the Headmaster's plan?

"Now, Dienstmann said that Grindelwald wasn't evil after all," Takushiki went on. "That evil things were happening anyway, whatever Grindelwald did, and if he exploited them a little what did it really matter? He did them reluctantly, Dienstmann said, and to avoid greater bloodshed."

Harry heard Snape's voice again. Must you take this potion and go ahead with your plan? And again, Goodbye, Headmaster. I don't know what the situation was, thought Harry, but Snape knew it. And perhaps he found a way to take advantage of it.

"Well, to exploit it, he had to cooperate with it," Dean Thomas said firmly. "And I don't believe for a minute the argument about bloodshed. He was only making himself rich!"

"Right. He was helping evil, miss. That's the same as doing it, isn't it?" Neville blushed and fell silent.

"What if you're bewitched by the Dark side, miss, no one can blame you then, can they?" That was Ron, thinking of Ginny, no doubt.

Takushiki turned to him. "That's why we're learning about spell detection and neutralisation this year, Ron, so none of you can get caught like that – unless you want to be."

Harry heard Hermione's voice. "Dienstmann said that Grindelwald was trying to do good by his experiments with the Dark as well, didn't he? That he was looking for a potion to strengthen magical powers. That would help people like Squibs, wouldn't it?"

"I think it's agreed the only powers he tried to increase were his own - but he never succeeded," Takushiki explained. "It's something that can't be done, any more than magical powers can be permanently decreased. They can be weakened temporarily but that's only a little, and it's only used in asylums where violent patients might harm themselves."

"Well," persisted Hermione, "if you can't weaken or strengthen someone's powers, can you share them around a bit more fairly? Wouldn't it make sense if people who had strong magical powers gave some of them to, say, Squibs?"

At those words, Ron's mouth dropped open – and Neville's, and Pansy's. Nearly the whole class fell into a stunned silence, leaving only the Muggle-born students looking at each other bewildered.

Professor Takushiki cleared her throat. "Class – Hermione obviously didn't mean anything by that, so calm down." She continued, "Hermione, you've just managed to blurt out the ultimate evil that any wizard or witch could attempt. I'm going to say the words only once: transferring magical power. It's far and away more…taboo…even than You-know-who's name. As far as anyone knows, it's impossible, and every attempt to do it over the centuries has been tragically unsuccessful and …has brought dreadful consequences. Somehow, even trying to do it messes about with the whole structure of matter near the attempt." She closed her eyes for a moment.

"You'll see it seventh year, when we study natural disasters, but for now..." she shook her head silently, "…we'll have to get back to Grindelwald. A nice safe topic." The class tittered nervously and began to settle down. "We have general agreement here that people are tempted into the Dark by various types of self-interest. But we're all self-interested sometimes, or stupid, or spiteful. Are we any different from Grindelwald?"

Neville put his hand up. "Most people know that those things are wrong. They don't keep on doing them and they're sorry."

The Defense mistress nodded. "Whereas those who go over to the Dark?..."

"They know what they're doing is wrong but they still do it anyway. They don't stop."

"Exactly." said Takushiki triumphantly. "No one is destined to be dark and one can always pull back along the path to it. But the first steps – and the last – along that road are always willing."

Harry sat and listened as Takushiki reminded them of Grindelwald's end: how Dumbledore had challenged him to a duel and Grindelwald had laughed and said that he would easily defeat so weak and unknown a wizard; how he had even sent Dumbledore a gravestone with his name carved on it. But when he had thought to surprise Dumbledore on the lonely island where he was then living, he had been overwhelmed and defeated, though Dumbledore modestly never told the whole story. But it was this that had made his reputation. He was the wizard who had defeated the Dark Lord in single combat .

Was that what Dumbledore had been talking about? Harry thought suddenly. Suppose Dumbledore had been preparing for another duel – but this time against Voldemort. "I intend to do a little traveling, take care of some personal business…" he had said. He remembered McGonagall's response: "Already?" when he had run to her for help as Voldemort attacked. But what would have made his opponent come early? Once again he felt the mysterious itching in his scar that he had felt when Snape walked past with the Headmaster's potion.

A few hours later the three friends stood outside Professor Takushiki's door. Dinner had been over in minutes as no one in the school was in the mood for talking, apart from "Are you feeling better, Harry?" and Lavender Brown's conviction that she had foreseen the whole thing in a teacup a month earlier. They had only had to wait to make sure that Takushiki had gone to the staff room with the other teachers. A meeting had been scheduled that night; Sprout had remarked during Herbology that it was expected to go long.

Hermione looked at Harry anxiously. "You're sure you feel all right?" she whispered. "Remember, Madam Pomfrey said she was counting on me to make sure you didn't overdo it."

Harry rolled his eyes in exasperation. "Hermione, when are you going to realise that the world will not collapse if you stop worrying about it!" he said sharply. "I'm fine! Let's go!"

They entered the office; Hermione quickly ignited the grate and each taking a handful of Floo powder from the ginger jar on Takushiki's desk, they threw it onto the flames and announced "Snape's office." There was a whoosh, an impression of whirling darkness, and they tumbled out into the dank chill of Snape's dungeon office.

"Lumos," whispered Harry. The room was so cold and still that he could hear the blood pounding in his ears. Through the gloom he could see the jars of specimens that Snape kept stacked up around the office; he hoped he was only imagining that a gristly white lump in one of them was beating.

"There are books on that top shelf there," pointed Hermione. They crept over.

"But there are at least fifty of them and they all look the same!" said Ron. "This is hopeless!"

Glancing up, Harry saw that most of the volumes were bound in black leather and covered with strange symbols. Few had titles on their spines.

Suddenly he had an idea. "Give me that chair," he said. Ron moved it over and he clambered up so that his head was level with the top shelf. Almost at once his scar began to pulse. The feeling got stronger and stronger as he ran his hand along the volumes stacked there until he came to one small book right at the end. His head reeled and he caught at the bookcase for support.

Hermione gasped. "Don't touch it – it might be enchanted!"

Harry shook his head to clear it. He stepped down, breathing hard. "You check it. It's…I don't know. I can't get near it."

Hermione took his place on the chair and poked at the book carefully with her wand. "Right. Oh, that's not so hard. Interrupte incantatem.." She stepped down. "Try it now."

Harry climbed up again. This time his scar itched furiously, just as it had outside Dumbledore's office. He gritted his teeth, prepared himself for the expected stabbing pain in the head, and pulled the book out.

The cover was coarse, more like untreated hide than cured leather, and it felt strangely hot to his hand. It was all he could do to stop himself from dropping it in disgust but he hopped down and tossed the book quickly onto Snape's empty desk.

"You'd better read it," he said to Hermione and stepped away a few paces to where the pain in his head was bearable.

Hermione picked it up gingerly and inspected the spine as Ron raised his wand to shine on it. "This could be it – it has the same strange mix of letters in the title." She opened it. Inside the front cover was a small label. "Property of the Institute of Potions," she read out. "Well, he shouldn't have kept it if it belongs to a library."

"Yeah, forget about being in league with the Dark, we'll just report him to the book police!" muttered Ron. "Get on with it!"

Even from a distance, Harry could see that the book was not printed but was more like a notebook. Its thick yellow pages were covered with writing and symbols in many different hands.

"It's very old," said Hermione, bending closer to it. "Some of this writing looks medieval. It's as if everyone who's ever owned it has added a new potions formula to it; but most of them are so complicated I can't even follow the introductions."

"That one's in English," said Ron, pointing over her shoulder. "Read it out."

"This be the potion called Upas Milk," Hermione began, "for it useth an single measure of the Upas tree's sap, which tree, many a traveller sayeth, will kill any, man or beast, who sleepeth beneath it, from the very poison fumes that rise off it, being like a steam or sweat. But he who will brave the mortal danger in getting and in using this sap may use it and so make such a draught as will certes weaken his enemy be he man or child, by the same dose; and most subtely, for the potion does not kill of itself but maketh the drinker drowsy and weak, so that whatsoever powers he possesseth run out of him like water from a broken bowl. No power of limb nor will doth it abate, but the magick vertu alone. It is a potion most suitable to be made by those of dark and melancholy humours."

There was silence for a moment. Finally Harry spoke. "A potion that makes you weak, so you can be attacked. That gives off dangerous fumes."

Ron paled. "Remember Snape coughing that night we went past his office? Peeves was asking him what he was brewing."

"I can't believe it." Hermione sagged into Snape's chair, near tears. "I can't believe he'd do that to Dumbledore, not after he trusted him. Oh, what are we going to do?"

"Let's get out of here," said Ron firmly. "Then I'll send an owl to my father."

Harry shook his head. "Who's going to believe us? Dumbledore said it himself last year – no one will take our word against Snape's. And even if your father does believe us, what can he do without evidence? We have to keep looking."

"What about the ingredients?" Ron asked. "The Institute of Potions would have records on whoever bought the restricted ones. We can even have a look round here right now."

Hermione brightened, and pulled a piece of paper toward her. "I'll make a list while you look. And there's a bookmark toward the back – shall I copy that page out, too?" She set her wand in a test tube rack and reached for a pen.

Harry snorted. "Probably something for his next victim! Go ahead, Hermione, it can't hurt." He crossed to the opposite wall, where hundreds of jars of every size were crowded together on the ancient oaken shelving. "Let's see: umber, umbilicus of tiger, uranyl acetate, urtica sativa, valeriana officinalis, vanadium pentoxide… hey, where is it?" His confusion was interrupted by a cry of triumph. He turned to see Ron brandishing a looseleaf notebook.

"It's his purchase record! We've got him now!" cried Ron. "October 18, Agrippinilla Alchemical Supply: freeze-dried squid ink, boomslang stomach lining, extract of venomous tentacula, tri-N-butyl phosphate, balm of Gilead, horseradish peroxidase, curare... Damn! It's not here either."

Hermione looked up. "It wouldn't be on the shelf," she said, one finger on the page. "There's a note in the margin saying that the sap has to be stored frozen. And if you bought Dark Arts ingredients, would you write them down on a list anyone could see? Anyway, the squid ink and tentacula extract match, and the Institute of Potions would have the alchemist's sales record. All we have to do is check."

A loud click came from the direction of the door, and they whirled in terror. Then it sounded again as the minute hand on Snape's wall clock moved past the hour. Harry breathed a sigh of relief. "Let's get out of here."

"Right." said Hermione, replacing the book on the shelf and tapping it with her wand. "Reverte incantatem. Is everything where it was?"

Harry and Ron nodded. Floo powder in hand, the three friends stepped into the fireplace and were gone.