Hathaway was so surprised he blew the match out and replaced the cigar in his pocket. 

Then the girl stepped briskly to one of the long tables and reached for a large volume that lay open on it. Hathaway was ahead of her. 'What are you going to do, Miss?'

Susan looked completely astonished. 'I'm just going to put it back on the shelves.'

'Why? What is it?' He looked down at the open page. I was there too, by then. I looked over his shoulder.

It was German. I can't read the language, but I can recognize it when I see it. The printing was small, and there were geometrical figures on the page with lines of letters attached at various places. I knew enough, too, to know those were chemical formulas.

I put my finger in the place, closed the book and looked at the backstrap. It said, 'Beilstein-Organische Chemie-Band VI-System Nummer 499-608.' I opened to the page again. It was 233 and the first words, just to give you an idea, were 4'-chlor-4-brom-2-nitro-diphenylather-C12H7O3 NClBr.

Hathaway was busy copying things down.

Professor Rodney was at the table too, which made four of us all gathered round the book.

The professor said in a cool voice, as though he were on a platform with a pointer in one hand and a piece of chalk in the other, 'This is a volume of Beilstein.' (He pronounced it Bile-shtine.) 'It's a kind of encyclopedia of organic compounds. It lists hundreds of thousands of them.'

'In this book?' demanded Hathaway.

This book is only one of more than sixty volumes and supplementary volumes. It is a tremendous German work which is years out of date because, first, organic chemistry is progressing at an ever-increasing pace and, second, because of the interference of politics and war. Even so, there is nothing even faintly approaching its usefulness in English. For all research men in organic chemistry, these volumes are an absolute necessity.'

The professor actually patted the book as he spoke, a fond pat. 'Before dealing with any unfamiliar compound,' he said, 'it is good practice to look it up in Beilstein. It will give you methods of preparation, properties, references, and so on. It acts as a starting point. The various compounds are listed according to a logical system which is clear but not obvious. I myself give several lectures in my course on organic syntheses which deal entirely with methods for finding a particular compound somewhere in the sixty volumes.'

I don't know how long he might have continued, but I wasn't there to learn organic syntheses and it was time to get down to cases. I said abruptly, 'Professor, I want to speak to you in your laboratory.'

I suppose I had some notion that cyanide was kept in a safe, that every bit of it was accounted for, that people had to sign out for it when they wanted some. I thought the question of opportunity to get some illicitly might supply what proof we needed.

And there I stood with a pound of it in my hand and the knowledge that anyone could have any amount for the asking, or without asking.

And he said thoughtfully, 'They used to call them the "Library Twins."'

I nodded. 'So?' 

'Only that it proves how superficial the judgment of most people is. There was nothing alike about them except the accident of hair and eyes. What happened in the library, Inspector?'

I told him Susan's story briefly and watched him.

He shook his head. 'I suppose you think the dead girl planned murder.' My thoughts weren't for sale at the moment. I said, 'Don't you?'

'No. She was incapable of it. Her attitude toward her duties was a pleasant and helpful one. Besides, why would she?'

'There's a student,' I said. 'Peter is his first name.'

'Peter van Norden,' he said at once. 'A reasonably bright student, but, somehow, worthless.'

'Girls look at these things differently, Professor. Both librarians were apparently interested. Susan may have been the more successful and Louella-Marie may have decided on direct measures.'

'And then proceeded to take the wrong cup?'

I said, 'People do queer things under tension.'

'Not this queer,' he said. 'One cup was left unsugared, so the murderess wasn't taking chances. Presumably even if she had not carefully memorized which cup was which, she could count on the sweetness to give it away. She could easily have avoided a fatal dose.'

I said dryly, 'Both girls usually took sugar. The dead girl was used to sweet tea. In the excitement the accustomed sweetness didn't ring a bell.'

'I don't believe it.'

'What's the alternative, Professor? The sugar was hocused after Mrs. Nettler's tea at one o'clock. Did Mrs. Nettler do it?'

He looked up sharply. 'What possible motive?'

I shrugged. 'She might have been afraid the girls were going to be taking her job away.' That's nonsense. She's retiring before the fall session begins.'

'You were there, Professor,' I said softly.

He took it in stride, to my surprise. 'Motive?' he said.

I said, 'You're not too old to have been interested in Louella-Marie, Professor. Suppose she had threatened to report some word or act of yours to the Dean.'

The Professor smiled bitterly. 'How did I manage to make sure the right girl got the cyanide? Why should one cup remain unsugared? I may have hocused the sugar but I didn't prepare the tea.'

I began to change my mind about Professor Rodney. He hadn't bothered to work up indignation or register shock.

He simply pointed out the logical weakness and let it go at that. I liked that.

I said, 'What do you think happened?'

He said, The mirror-image. The reverse. I think the survivor told the truth inside out. Suppose it was

Louella-Marie who was getting the boy and Susan who didn't like it, rather than the reverse. Suppose it was Susan who for once was preparing the tea and Louella-Marie who was at the front desk rather than the reverse. In that case, the girl who prepared the tea would have taken the right cup and remained safe. Everything would be logical instead of ridiculously improbable.'

That did it. The man had come to the same conclusion I had and so I had to like him after all. I have a habit of feeling soft toward guys who agree with me. It comes of being Homo sapiens, I think.

I said, 'We've got to prove that beyond reasonable doubt. How? I'd come up here, hoping to prove someone had had access to potassium cyanide and others had not. That's out. Everyone had access. Now what?'

The professor said, 'Check on which girl was really at the desk at two o'clock when the tea was being prepared.'

It was obvious to me that the professor read detective stories and had faith in witnesses. I didn't, but I got up anyway.

'All right, Professor. I'll do that.'

The professor rose also. He said urgently, 'May I be present?'

I considered. 'Why? Your responsibilities to the Dean?'

'In a way. I would like to see a quick, clean end to this.'

I said, 'Come along, if you think that will help.'

Ed Hathaway was waiting for me when I came down. He was sitting in an empty library. He said. 'I got it.'

'Got what?' I wanted to know.

'How it happened. I figured it out by deduction.'

'Oh?'

He was paying no attention to Professor Rodney. The cyanide had to be smuggled in. By whom? By the joker in the deck, the outsider, the guy with the accent-whatzis-name.'

He started scrabbling through a series of cards on which he had filed information on the various presumably innocent bystanders.

I knew who he meant so I said, 'All right, never mind the name. What's in a name? Go on'-which shows that I can be as unbright as anyone.

'All right. The foreigner comes in with the cyanide in a little envelope. He tapes the envelope to a page in the German book, that organish whatzisname with all the volumes…' The professor and I both nodded.