one of breaking the news to Mrs. Northrop. I shall pass 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 shaggy 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-glass 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 superstition that whoever embraces it will die before the sun goes down.
“‘From the subterraneo is said to lead a long, underground passage 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.”
“I’d like to know what is in that,” remarked Craig, turning it over and over.
He appeared to be considering something, for he rose suddenly and shoved the letter into his pocket.
I followed, and a few moments later, across the campus in his laboratory, he was working quickly over an X-ray apparatus. He had placed the letter in it.
“These are what are known as ‘low’ tubes,” he explained. “They give out ‘soft rays.’” He continued to work for a few moments, then handed me the letter.
“Now, Walter,” he said, “if you will just hurry back to the museum and replace that letter, I think I will have something that will astonish you—though whether it will have any bearing on the case, remains to be seen.”
“What is it?” I asked, a few minutes later, when I had rejoined him, after returning the letter. He was poring intently over what looked like a negative.
“The possibility of reading the contents of documents inclosed in a sealed envelope,” he replied, still studying the shadowgraph closely, “has already been established by the well-known English scientist, Doctor Hall Edwards. He has been experimenting with the method of using X-rays recently discovered by a German scientist, by which radiographs of very thin substances, such as a sheet of paper, a leaf, an insect’s body, may be obtained. These thin substances through which the rays used formerly to pass without leaving an impression, can now be radiographed.”
I looked carefully as he traced out something on the negative. On it was easily possible, following his guidance, to read the words inscribed on the sheet of paper inside. So admirably defined were all the details that even the gum on the envelope and the edges of the sheet of paper inside the envelope could be distinguished.
“Any letter written with ink having a mineral basis can be radiographed,” added Craig. “Even when the sheet is folded in the usual way, it is possible by taking a radiograph stereoscopically, to distinguish the writing, every detail standing out in relief. Besides, it can be greatly magnified, which aids in deciphering it if it is indistinct or jumbled up. Some of it looks like mirror writing. Ah,” he added, “here’s something interesting!”
Together we managed