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 to trace out the contents of several paragraphs, of which the significant parts were as follows:

I am expecting that my friend Senora Herreria will be in New York by the time you receive this, and should she call on you, I know you will accord her every courtesy. She has been in Mexico City for a few days, having just returned from Mitla, where she met Professor Northrop. It is rumored that Professor Northrop has succeeded in smuggling out of the country a very important stone bearing an inscription which, I understand, is of more than ordinary interest. I do not know anything definite about it, as Senora Herreria is very reticent on the matter, but depend on you to find out if possible and let me know of it.

According to the rumors and the statements of the senora, it seems that Northrop has taken an unfair advantage of the situation down in Oaxaca, and I suppose she and others who know about the inscription feel that it is really the possession of the government.

You will find that the senora is an accomplished antiquarian and scholar. Like many others down here just now, she has a high regard for the Japanese. As you know, there exists a natural sympathy between some Mexicans and Japanese, owing to what is believed to be a common origin of the two races.

In spite of the assertions of many to the contrary, there is little doubt left in the minds of students that the Indian races which have peopled Mexico were of Mongolian stock. Many words in some dialects are easily understood by Chinese immigrants. A secretary of the Japanese legation here was able recently to decipher old Mixtec inscriptions found in the ruins of Mitla.

Senora Herreria has been much interested in establishing the relationship and, I understand, is acquainted with a Japanese curio dealer in New York who recently visited Mexico for the same purpose. I believe that she wishes to collaborate with him on a monograph on the subject, which is expected to have a powerful effect on the public opinion both here and at Tokyo.

In regard to the inscription which Northrop has taken with him, I rely on you to keep me informed. There seems to be a great deal of mystery connected with it, and I am simply hazarding a guess as to its nature. If it should prove to be something which might interest either the Japanese or ourselves, you can see how important it may be, especially in view of the forthcoming mission of General Francisco to Tokyo.

Very sincerely yours,

DR. EMILIO SANCHEZ, Director.

"Bernardo is a Mexican," I exclaimed, as Kennedy finished reading, "and there can be no doubt that the woman he mentioned was this Senora Herreria."

Kennedy said nothing, but seemed to be weighing the various paragraphs in the letter.

"Still," I observed, "so far, the only one against whom we have any direct suspicion in the case is the shaggy Russian, whoever he is."

"A man whom Bernardo says looked like a Russian," corrected Craig.

He was pacing the laboratory restlessly.

"This is becoming quite an international affair," he remarked finally, pausing before me, his hat on. "Would you like to relax your mind by a little excursion among the curio shops of the city? I know something about Japanese curios—more, perhaps, than I do of Mexican. It may amuse us, even if it doesn't help in solving the mystery. Meanwhile, I shall make arrangements for shadowing Bernardo. I want to know just how he acts after he reads that letter."

He paused long enough to telephone his instructions to an uptown detective agency which could be depended on for such mere routine work, then joined me with the significant remark: "Blood is thicker than water, anyhow, Walter. Still, even if the Mexicans are influenced by sentiment, I hardly think that would account for the interest of our friends from across the water in the matter."

I do not know how many of the large and small curio shops of the city we visited that afternoon. At another time, I should have enjoyed the visits immensely, for anyone seeking articles of beauty will find the antique shops of Fifth and Fourth Avenues and the side streets well worth visiting.

We came, at length, to one, a small, quaint, dusty rookery, down in a basement, entered almost directly from the street. It bore over the door a little gilt sign which read simply, "Sato's."

As we entered, I could not help being impressed by the wealth of articles in beautiful cloisonne enamel, in mother-of-pearl, lacquer, and champleve. There were beautiful little koros, or incense burners, vases, and teapots. There were enamels incrusted, translucent, and painted, works of the famous Namikawa, of Kyoto, and Namikawa, of Tokyo. Satsuma vases, splendid and rare examples of the potter's art, crowded gorgeously embroidered screens depicting all sorts of brilliant scenes, among others the sacred Fujiyama rising in the stately distance. Sato himself greeted us with a ready smile and bow.

"I am just looking for a few things to add to my den," explained Kennedy, adding, "nothing in particular, but merely whatever happens to strike my fancy."

"Surely, then, you have come to the right shop," greeted Sato. "If there is anything that interests you, I shall be glad to show it."

"Thank you," replied Craig. "Don't let me trouble you with your other customers. I will call on you if I see anything."

For several minutes, Craig and I busied ourselves looking about, and we did not have to feign interest, either.

"Often things are not as represented," he whispered to me, after a while, "but a connoiseur can tell spurious goods. These are the real thing, mostly."

"Not one in fifty can tell the difference," put in the voice of Sato, at his elbow.

"Well, you see I happen to know," Craig replied, not the least disconcerted. "You can't always be too sure."

A laugh and a shrug was Sato's answer. "It's well all are not so keen," he said, with a frank acknowledgment that he was not above sharp practices.

I glanced now and then at the expressionless face of the curio dealer. Was it merely the natural blankness of his countenance that impressed me, or was there, in fact, something deep and dark hidden in it, something of "East is East and West is West" which I did not and could not understand? Craig was admiring the bronzes. He had paused before one, a square metal fire-screen of odd design, with the title on a card, "Japan Gazing at the World."

It represented Japan as an eagle, with beak and talons of burnished gold, resting on a rocky island about which great waves dashed. The bird had an air of dignity and conscious pride in its strength, as it looked out at the world, a globe revolving in space.

"Do you suppose there is anything significant in that?" I asked, pointing to the continent of North America, also in gold and prominently in view.

"Ah, honorable sir," answered Sato, before Kennedy could reply, "the artist intended by that to indicate Japan's friendliness for America and America's greatness."

He was inscrutable. It seemed as if he were watching our every move, and yet it was done with a polite cordiality that could not give offense.

Behind some bronzes of the Japanese Hercules destroying the demons and other mythical heroes was a large alcove, or tokonoma, decorated with peacock, stork, and crane panels. Carvings and lacquer added to the beauty of it. A miniature chrysanthemum garden heightened the illusion. Carved hinoki wood framed the panels, and the roof was supported by columns in the old Japanese style, the whole being a compromise between the very simple and quiet and the polychromatic. The dark woods, the lanterns, the floor tiles of dark red, and the cushions of rich gold and yellow were most alluring. It had the genuine fascination of the Orient.

"Will the gentlemen drink a little sake?" Sato asked politely.

Craig thanked him and said that we would.

"Otaka!" Sato called.

A peculiar, almost white-skinned attendant answered, and a moment later produced four cups and poured out the rice brandy, taking his own quietly, apart from us. I watched him drink, curiously. He took the cup; then, with a long piece of carved wood, he dipped into the sake, shaking a few drops on the floor to the four quarters. Finally, with a deft sweep, he lifted his heavy mustache with the piece of wood and drank off the draft almost without taking breath.

He was a peculiar man of middle height, with a shock of dark, tough, woolly hair, well formed and not bad-looking, with a robust general physique, as if his ancestors had been meat eaters. His forehead was narrow and sloped backward; the cheekbones were prominent; nose hooked, broad and wide, with strong nostrils; mouth large, with thick lips, and not very prominent chin. His eyes were perhaps the most noticeable feature. They were dark gray, almost like those of a European.

As Otaka withdrew with the empty cups, we rose to continue our inspection of the wonders of the shop. There were ivories of all descriptions. Here was a two-handled sword, with a very large ivory handle, a weirdly carved scabbard, and wonderful steel blade. By the expression of Craig's face, Sato knew that he had made a sale.

Craig had been rummaging among some warlike instruments which Sato, with the instincts of a true salesman, was now displaying, and had picked up a bow. It was short, very strong, and made of pine wood. He held it horizontally and twanged the string. I looked up in time to catch a pleased expression on the face of Otaka.

"Most people would have held it the other way," commented Sato.

Craig said nothing, but was examining an arrow, almost twenty inches long and thick, made of cane, with a point of metal very sharp but badly fastened. He fingered the deep blood groove in the scooplike head of the arrow and looked at it carefully.

"I'll take that," he said, "only I wish it were one with the regular reddish-brown lump in it."

"Oh, but, honorable sir," apologized Sato, "the Japanese law prohibits that, now. There are few of those, and they are very valuable."

"I suppose so," agreed Craig. "This will do, though. You have a wonderful shop here, Sato. Some time, when I feel richer, I mean to come in again. No, thank you, you need not send them; I'll carry them."

We bowed ourselves out, promising to come again when Sato received a new consignment from the Orient which he was expecting.

"That other Jap is a peculiar fellow," I observed, as we walked along uptown again.

"He isn't a Jap," remarked Craig. "He is an Ainu, one of the aborigines who have been driven northward into the island of Yezo."

"An Ainu?" I repeated.

"Yes. Generally thought, now, to be a white race and nearer of kin to Europeans than Asiatics. The Japanese have pushed them northward and are now trying to civilize them. They are a dirty, hairy race, but when they are brought under civilizing influences they adapt themselves to their environment and make very good servants. Still, they are on about the lowest scale of humanity."

"I thought Otaka was very mild," I commented.

"They are a most inoffensive and peaceable people usually," he answered, "good-natured and amenable to authority. But they become dangerous when driven to despair by cruel treatment. The Japanese government is very considerate of them—but not all Japanese are."