"He removed his shoes--either to help in climbing or to prevent noise--ah--here's the foot! Strange--see how small it is--and broad, how prehensile the toes--almost like fingers. Surely that foot could never have been encased in American shoes all its life. I shall make plaster casts of these, to preserve later."

He was still scouting about on hands and knees in the dampness of the rhododendrons. Suddenly he reached his long arm in among the shrubs and picked up a little reed stick. On the end of it was a small cylinder of buff brown.

He looked at it curiously, dug his nail into the soft ma.s.s, then rubbed his nail over the tip of his tongue gingerly.

With a wry face, as if the taste were extremely acrid, he moistened his handkerchief and wiped off his tongue vigorously.

"Even that minute particle that was on my nail makes my tongue tingle and feel numb," he remarked, still rubbing. "Let us go back again. I want to see Bernardo."

"Had he any visitors during the day?" queried Kennedy, as he reentered the ghastly little room, while the curator stood outside, completely unnerved by the tragedy which had been so close to him without his apparently knowing it. Kennedy was squeezing out from the little wound on Northrop's neck a few drops of liquid on a sterilized piece of gla.s.s.

"No; no one," Bernardo answered, after a moment.

"Did you see anyone in the museum who looked suspicious?" asked Kennedy, watching Bernardo's face keenly.

"No," he hesitated. "There were several people wandering about among the exhibits, of course. One, I recall, late in the afternoon, was a little dark-skinned woman, rather good-looking."

"A Mexican?"

"Yes, I should say so. Not of Spanish descent, though. She was rather of the Indian type. She seemed to be much interested in the various exhibits, asked me several questions, very intelligently, too. Really, I thought she was trying to--er--flirt with me."

He shot a glance at Craig, half of confession, half of embarra.s.sment.

"And--oh, yes--there was another--a man, a little man, as I recall, with s.h.a.ggy hair. He looked like a Russian to me. I remember, because he came to the door, peered around hastily, and went away. I thought he might have got into the wrong part of the building and went to direct him right--but before I could get out into the hall, he was gone. I remember, too, that, as I turned, the woman had followed me and soon was asking other questions--which, I will admit--I was glad to answer."

"Was Northrop in his room while these people were here?"

"Yes; he had locked the door so that none of the students or visitors could disturb him."

"Evidently the woman was diverting your attention while the man entered Northrop's room by the window," ruminated Craig, as we stood for a moment in the outside doorway.

He had already telephoned to our old friend Doctor Leslie, the coroner, to take charge of the case, and now was ready to leave. The news had spread, and the janitor of the building was waiting to lock the campus door to keep back the crowd of students and others.

Our next duty was the painful one of breaking the news to Mrs.

Northrop. I shall pa.s.s it over. Perhaps no one could have done it more gently than Kennedy. She did not cry. She was simply dazed. Fortunately her mother was with her, had been, in fact, ever since Northrop had gone on the expedition.

"Why should anyone want to steal tablets of old Mixtec inscriptions?" I asked thoughtfully, as we walked sadly over the campus in the direction of the chemistry building. "Have they a sufficient value, even on appreciative Fifth Avenue, to warrant murder?"

"Well," he remarked, "it does seem incomprehensible. Yet people do just such things. The psychologists tell us that there is a veritable mania for possessing such curios. However, it is possible that there may be some deeper significance in this case," he added, his face puckered in thought.

Who was the mysterious Mexican woman, who the s.h.a.ggy Russian? I asked myself. Clearly, at least, if she existed at all, she was one of the millions not of Spanish but of Indian descent in the country south of us. As I reasoned it out, it seemed to me as if she must have been an accomplice. She could not have got into Northrop's room either before or after Doctor Bernardo left. Then, too, the toe-and shoe-prints were not hers. But, I figured, she certainly had a part in the plot.

While I was engaged in the vain effort to unravel the tragic affair by pure reason, Kennedy was at work with practical science.

He began by examining the little dark cylinder on the end of the reed.

On a piece of the stuff, broken off, he poured a dark liquid from a brown-gla.s.s bottle. Then he placed it under a microscope.

"Microscopically," he said slowly, "it consists almost wholly of minute, clear granules which give a blue reaction with iodine. They are starch. Mixed with them are some larger starch granules, a few plant cells, fibrous matter, and other foreign particles. And then, there is the substance that gives that acrid, numbing taste." He appeared to be vacantly studying the floor.

"What do you think it is?" I asked, unable to restrain myself.

"Aconite," he answered slowly, "of which the active principle is the deadly poisonous alkaloid, aconitin."

He walked over and pulled down a well-thumbed standard work on toxicology, turned the pages, then began to read aloud:

Pure aconitin is probably the most actively poisonous substance with which we are acquainted and, if administered hypodermically, the alkaloid is even more powerfully poisonous than when taken by the mouth.

As in the case of most of the poisonous alkaloids, aconitin does not produce any decidedly characteristic post-mortem appearances. There is no way to distinguish it from other alkaloids, in fact, no reliable chemical test. The physiological effects before death are all that can be relied on.

Owing to its exceeding toxic nature, the smallness of the dose required to produce death, and the lack of tests for recognition, aconitin possesses rather more interest in legal medicine than most other poisons.

It is one of the few substances which, in the present state of toxicology, might be criminally administered and leave no positive evidence of the crime. If a small but fatal dose of the poison were to be given, especially if it were administered hypodermically, the chances of its detection in the body after death would be practically none.

CHAPTER XI

THE "PILLAR OF DEATH"

I was looking at him fixedly as the diabolical nature of what must have happened sank into my mind. Here was a poison that defied detection. I could see by the look on Craig's face that that problem, alone, was enough to absorb his attention. He seemed fully to realize that we had to deal with a criminal so clever that he might never be brought to justice.

An idea flashed over me.

"How about the letters?" I suggested.

"Good, Walter!" he exclaimed.

He untied the package which Mrs. Northrop had given him and glanced quickly over one after another of the letters.

"Ah!" he exclaimed, fairly devouring one dated at Mitla. "Listen--it tells about Northrop's work and goes on:

"'I have been much interested in a cavern, or subterraneo, here, in the shape of a cross, each arm of which extends for some twelve feet underground. In the center it is guarded by a block of stone popularly called "the Pillar of Death." There is a superst.i.tion that whoever embraces it will die before the sun goes down.

"'From the subterraneo is said to lead a long, underground pa.s.sage across the court to another subterranean chamber which is full of Mixtec treasure. Treasure hunters have dug all around it, and it is said that two old Indians, only, know of the immense amount of buried gold and silver, but that they will not reveal it.'"

I started up. Here was the missing link which I had been waiting for.

"There, at least, is the motive," I blurted out. "That is why Bernardo was so reticent. Northrop, in his innocence of heart, had showed him that inscription."

Kennedy said nothing as he finally tied up the little packet of letters and locked it in his safe. He was not given to hasty generalizations; neither was he one who clung doggedly to a preconceived theory.

It was still early in the afternoon. Craig and I decided to drop into the museum again in order to see Doctor Bernardo. He was not there and we sat down to wait.

Just then the letter box in the door clicked. It was the postman on his rounds. Kennedy walked over and picked up the letter.

The postmark bore the words, "Mexico City," and a date somewhat later than that on which Northrop had left Vera Cruz. In the lower corner, underscored, were the words, "Personal--Urgent."