At that moment a man strolled past them, a big stout fellow, showily dressed in a check suit; and he gravely took off his hat to Dr Porhoet.  The doctor smiled and returned the salute.

‘Who is your fat friend?’ asked Arthur.

‘That is a compatriot of yours.  His name is Oliver Haddo.’

‘Art-student?’ inquired Arthur, with the scornful tone he used when referring to those whose walk in life was not so practical as his own.

’Not exactly.  I met him a little while ago by chance.  When I was getting together the material for my little book on the old alchemists I read a great deal at the library of the Arsenal, which, you may have heard, is singularly rich in all works dealing with the occult sciences.’

Burden’s face assumed an expression of amused disdain.  He could not understand why Dr Porhoet occupied his leisure with studies so profitless.  He had read his book, recently published, on the more famous of the alchemists; and, though forced to admire the profound knowledge upon which it was based, he could not forgive the waste of time which his friend might have expended more usefully on topics of pressing moment.

‘Not many people study in that library,’ pursued the doctor, ’and I soon knew by sight those who were frequently there.  I saw this gentleman every day.  He was immersed in strange old books when I arrived early in the morning, and he was reading them still when I left, exhausted.  Sometimes it happened that he had the volumes I asked for, and I discovered that he was studying the same subjects as myself.  His appearance was extraordinary, but scarcely sympathetic; so, though I fancied that he gave me opportunities to address him, I did not avail myself of them.  One day, however, curiously enough, I was looking up some point upon which it seemed impossible to find authorities.  The librarian could not help me, and I had given up the search, when this person brought me the very book I needed.  I surmised that the librarian had told him of my difficulty.  I was very grateful to the stranger.  We left together that afternoon, and our kindred studies gave us a common topic of conversation.  I found that his reading was extraordinarily wide, and he was able to give me information about works which I had never even heard of.  He had the advantage over me that he could apparently read, Hebrew as well as Arabic, and he had studied the Kabbalah in the original.’

‘And much good it did him, I have no doubt,’ said Arthur.  ’And what is he by profession?’

Dr Porhoet gave a deprecating smile.

’My dear fellow, I hardly like to tell you.  I tremble in every limb at the thought of your unmitigated scorn.’

‘Well?’

’You know, Paris is full of queer people.  It is the chosen home of every kind of eccentricity.  It sounds incredible in this year of grace, but my friend Oliver Haddo claims to be a magician.  I think he is quite serious.’