Bella continues to search the library for materials about the Hundred Years' War.

Elena was bored and leaned on the table to empty herself.

Monsieur Pierre was carefully tidying up the shelves.

From time to time, there were a few canary chirps outside the window, and occasionally three or two students came to look for books, but they left quickly.

I thought a peaceful day would end like this, but in the afternoon, Officer Christopher and Professor Pascal reappeared in the library.

"Pierre, please help me find a book on alchemy materials." Professor Pascal said.

Pierre answered and left the front desk.

"Huh? Alchemy?" Elena sprang up from the table, "I'm familiar with this!"

Elena came from a family of alchemists, so of course she knew a lot about this.

"We've found some strange powder on the shore." Officer Christopher said, showing her the powdered handkerchief, "Do you know what this is?"

Elena took the handkerchief and saw that there was some yellow-green powder on it.

She leaned in and sniffed again.

"It's 'cold light.'" "It glows blue-green in the night sky, and it burns easily."

The "cold light", or phosphorus, was discovered by a German merchant in the process of alchemy. This substance can emit light in a dark environment. Unlike today's incandescent lamps, this light is a low-temperature light that does not heat up or ignite other substances, so it is called "cold light" (the Latin word Phosphorun means "cold light").

"So, is this ghost incident really deliberate?" Christopher said with a frown.

"I think that by tying it up with a rod and a line and hanging it in mid-air, we can create the scene that Mr. Robert saw yesterday on the river." Professor Pascal analyzed.

"Oh, it's boring." Elena sighed, "I thought there was going to be something interesting. It turned out that the ghost turned out to be fake. The problem of books has not been solved."

"Huh? What is the problem with books?" Professor Pascal asked, confused.

"It was this morning when Bella found a blank book on the top of the shelf with nothing written on it." "But Monsieur Pierre doesn't know where it came from."

"Blank book?" Professor Pascal looked interested, too, "Can you show me?"

"Okay." Elena said, standing up and walking towards the bookshelf.

"Then I'll leave." Officer Christopher said, "I'll have to investigate who did this prank."

"If you need anything, you can come back to me." Professor Pascal said.

"No problem." Christopher waved his hand and turned to leave the library.

Elena took the strange book.

"Give." She handed the book to Professor Pascal.

The professor took the thick book and weighed it in his hand.

"It's not a small weight."

As he spoke, he placed the book on the table, and slowly opened the first page. The note with the words written on it came into view.

"It's ......," Professor Pascal picked up the note and read it carefully.

"I've been here before." Elena explained.

"This should be the Saint Jeanne Darke, right?" Professor Pascal said, "I think she was an Orleans girl who led the French army in the Reconquista during the Hundred Years' War. Not long ago, he was canonized by the Catholic Church."

"That's right," Bella walked over to hear them chatting, "I guess I'm a national hero of France."

"In that case, it shouldn't be unusual for someone to write a biography about her." Pascal said.

"You mean this book is a biography?" Elena asked.

"Huh? Isn't it?" Pascal said, "That's what most people think, right?"

Otherwise, how could there be such a note?

"But there's not a word in it?"

"Hmm......" Pascal flipped through the book, "yes, indeed."

It's all blank.

"Did you forget to print it?" Elena said suspiciously.

"How is that possible?" Professor Pascal said, "Usually books are printed before they are bound together, right?"

Who would bind a pile of blank paper?

"But aren't all publications now machine-run? From paper printing to bookbinding, it may not go through human hands."

"But machine production has only been around since the Industrial Revolution in recent years, right?" Bella said, "It's been at least a hundred years since I looked at this book."

In other words, there are no errors in machine production.

Taking a step back, even if it was a printing error, it was a book without words, so why would it still be placed in a library?

If you want to donate to the library, you should go through a careful check.

Moreover, this book has not even been seen by Monsieur Pierre, and this is the strangest part.

"There's another possibility." Professor Pascal said, "Could it be that this book was written in a way that we can't see?"

"How we can't see it?" Elena was puzzled.

"I know a person who often studies the reactions and changes between substances." "According to him, there are many ways to make something that is otherwise transparent appear a certain color."

"So, if someone writes something on it with colorless ink, they can make the text appear in a certain way?" Elena asked.

"Yes, but that's just my guess. Whether or not someone actually made the book in this way is uncertain. And even if someone does, there's no guarantee that we'll be able to find a way to make the text visible." Professor Pascal said.

"So, is there any way to know for sure?"

"Well...... Professor Pascal thought for a moment, then replied, "I might have to ask my friend to find out."

"The guy you just mentioned?"

"That's right. However, he seems to have just become a tax collector recently, and he may be busy, so I don't know if he is free now." "And I don't know how his research is going."

"Can you please help me ask?"

"No problem." As he spoke, Professor Pascal asked Mr. Pierre, who was next to him, "Can I take this book out there?"

"Uh......" Pierre hesitated for a moment, then said, "Yes, yes, but the book is not in the book, or ...... Could you please come and register manually?"

I don't know where the book came from, but after all, it appeared in the library, and the procedures still had to be done.