'Enfin, I abandon it!' I said aloud. 'There is not a clue anywhere! Everything is perfectly normal.'

As I said the words, my eyes fell on a large box of chocolates standing on a table near by, and my heart gave a leap. It might not be a clue to M. Déroulard's death, but here at least was something that was not normal. I lifted the lid. The box was full, untouched; not a chocolate was missing - but that only made the peculiarity that had caught my eye more striking. For, see you, Hastings, while the box itself was pink, the lid was blue. Now, one often sees a blue ribbon on a pink box, and vice versa, but a box of one colour, and a lid of another - no; decidedly - ça ne se voit jamais!

I did not as yet see that this little incident was of any use to me, yet I determined to investigate it as being out of the ordinary. I rang the bell for François, and asked him if his late master had been fond of sweets. A faint melancholy smile came to his lips.

'Passionately fond of them, monsieur. He would always have a box of chocolates in the house. He did not drink wine of any kind, you see.'

'Yet this box has not been touched?' I lifted the lid to show him.

'Pardon, monsieur, but that was a new box purchased on the day of his death, the other being nearly finished.'

'Then the other box was finished on the day of his death,' I said slowly.

'Yes, monsieur, I found it empty in the morning and threw it away.'

'Did M. Déroulard eat sweets at all hours of the day?'

'Usually after dinner, monsieur.'

I began to see light.

'François,' I said, 'you can be discreet?'

'If there is need, monsieur.'

'Bon! Know, then, that I am of the police. Can you find me that other box?'

'Without doubt, monsieur. It will be in the dustbin.'

He departed, and returned in a few minutes with a dust-covered object. It was the duplicate of the box I held, save for the fact that this time the box was blue and the lid was pink. I thanked François, recommended him once more to be discreet, and left the house in the Avenue Louise without more ado.

Next day I called upon the doctor who had attended M. Déroulard. With him I had a difficult task. He entrenched himself prettily behind a wall of learned phraseology, but I fancied that he was not quite as sure about the case as he would like to be.

'There have been many curious occurrences of the kind,' he observed, when I had managed to disarm him somewhat. 'A sudden fit of anger, a violent emotion - after a heavy dinner, c'est entendu - then, with an access of rage, the blood flies to the head, and pst! - there you are!'

'But M. Déroulard had had no violent emotion.'