He said it with a quiet confidence which I could not gainsay, although I did not understand.  However, mostly with the official aid of Doctor Leslie, I followed out his instructions, and it was indeed a strange party that assembled that night.  There were Doctor Bernardo; Sato, the curio dealer; Otaka, the Ainu, and ourselves.  Mrs. Northrop, of course, could not come.

“Mexico,” began Craig, after he had said a few words explaining why he had brought us together, “is full of historical treasure.  To all intents and purposes, the government says, ‘Come and dig.’  But when there are finds, then the government swoops down on them for its own national museum.  The finder scarcely gets a chance to export them.  However, now seemed to be the time to Professor Northrop to smuggle his finds out of the country.

“But evidently it could not be done without exciting all kinds of rumors and suspicions.  Stories seem to have spread far and fast about what he had discovered.  He realized the unsettled condition of the country—­perhaps wanted to confirm his reading of a certain inscription by consultation with one scholar whom he thought he could trust.  At any rate, he came home.”

Kennedy paused, making use of the silence for emphasis.  “You have all read of the wealth that Cortez found in Mexico.  Where are the gold and silver of the conquistadores?  Gone to the melting pot, centuries ago.  But is there none left?  The Indians believe so.  There are persons who would stop at nothing—­even at murder of American professors, murder of their own comrades, to get at the secret.”

He laid his hand almost lovingly on his powerful little microscope as he resumed on another line of evidence.

“And while we are on the subject of murders, two very similar deaths have occurred,” he went on.  “It is of no use to try to gloss them over.  Frankly, I suspected that they might have been caused by aconite poisoning.  But, in the case of such poisoning, not only is the lethal dose very small but our chemical methods of detection are nil.  The dose of the active principle, aconitin nitrate, is about one six-hundredth of a grain.  There are no color tests, no reactions, as in the case of the other organic poisons.”

I wondered what he was driving at.  Was there, indeed, no test?  Had the murderer used the safest of poisons—­one that left no clue?  I looked covertly at Sato’s face.  It was impassive.  Doctor Bernardo was visibly uneasy as Kennedy proceeded.  Cool enough up to the time of the mention of the treasure, I fancied, now, that he was growing more and more nervous.

Craig laid down on the table the reed stick with the little darkened cylinder on the end.

“That,” he said, “is a little article which I picked up beneath Northrop’s window yesterday.  It is a piece of anno-noki, or bushi.”  I fancied I saw just a glint of satisfaction in Otaka’s eyes.