We bent over. The writing signed "Thurston" on sheet number one was faint, almost imperceptible, but on paper number two, in black letters, appeared what Kennedy had written: "Dear Harris: Since we agreed to disagree we have at least been good friends."

"It is like the start of the subst.i.tuted letter, and the other is like the missing note," gasped Leland in a daze.

"Yes," said Kennedy quickly. "Leland, no one entered your office. No one stole the Thurston note. No one subst.i.tuted the Lytton letter.

According to your own story, you took them out of the safe and left them in the sunlight all day. The process that had been started earlier in ordinary light, slowly, was now quickly completed. In other words, there was writing which would soon fade away on one side of the paper and writing which was invisible but would soon appear on the other.

"For instance, quinoline rapidly disappears in sunlight. Starch with a slight trace of iodine writes a light blue, which disappears in air.

It was something like that used in the Thurston letter. Then, too, silver nitrate dissolved in ammonia gradually turns black as it is acted on by light and air. Or magenta treated with a bleaching-agent in just sufficient quant.i.ty to decolorise it is invisible when used for writing. But the original color reappears as the oxygen of the air acts upon the pigment. I haven't a doubt but that my a.n.a.lyses of the inks are correct and on one side quinoline was used and on the other nitrate of silver. This explains the inexplicable disappearance of evidence incriminating one person, Thurston, and the sudden appearance of evidence incriminating another, Dr. Dixon. Sympathetic ink also accounts for the curious circ.u.mstance that the Lytton letter was folded up with the writing apparently outside. It was outside and unseen until the sunlight brought it out and destroyed the other, inside, writing--a chance, I suspect, that was intended for the police to see after it was completed, not for the defence to witness as it was taking place."

We looked at each other aghast. Thurston was nervously opening and shutting his lips and moistening them as if he wanted to say something but could not find the words.

"Lastly," went on Craig, utterly regardless of Thurston's frantic efforts to speak, "we come to the note that was discovered so queerly crumpled up in the jar of ammonia on Vera Lytton's dressing-table. I have here a cylindrical gla.s.s jar in which I place some sal-ammoniac and quicklime. I will wet it and heat it a little. That produces the pungent gas of ammonia.

"On one side of this third piece of paper I myself write with this mercurous nitrate solution. You see, I leave no mark on the paper as I write. I fold it up and drop it into the jar--and in a few seconds withdraw it. Here is a very quick way of producing something like the slow result of sunlight with silver nitrate. The fumes of ammonia have formed the precipitate of black, mercurous nitrate, a very distinct black writing which is almost indelible. That is what is technically called invisible rather than sympathetic ink."

We leaned over to read what he had written. It was the same as the note incriminating Dixon:

This will cure your headache.

Dr. Dixon.

A servant entered with a telegram from New York. Scarcely stopping in his exposure, Kennedy tore it open, read it hastily, stuffed it into his pocket, and went on.

"Here in this fourth bottle I have an acid solution of iron chloride, diluted until the writing is invisible when dry," he hurried on. "I will just make a few scratches on this fourth sheet of paper--so. It leaves no mark. But it has the remarkable property of becoming red in vapor of sulpho-cyanide. Here is a long-necked flask of the gas, made by sulphuric acid acting on pota.s.sium sulpho-cyanide. Keep back, Dr.

Waterworth, for it would be very dangerous for you to get even a whiff of this in your condition. Ah! See--the scratches I made on the paper are red."

Then hardly giving us more than a moment to let the fact impress itself on our minds, he seized the piece of paper and dashed it into the jar of ammonia. When he withdrew it, it was just a plain sheet of white paper again. The red marks which the gas in the flask had brought out of nothingness had been effaced by the ammonia. They had gone and left no trace.

"In this way I can alternately make the marks appear and disappear by using the sulpho-cyanide and the ammonia. Whoever wrote this note with Dr. Dixon's name on it must have had the doctor's reply to the Thurston letter containing the words, 'This will not cure your headache.' He carefully traced the words, holding the genuine note up to the light with a piece of paper over it, leaving out the word 'not'

and using only such words as he needed. This note was then destroyed.

"But he forgot that after he had brought out the red writing by the use of the sulpho-cyanide, and though he could count on Vera Lytton's placing the note in the jar of ammonia and hence obliterating the writing, while at the same time the invisible writing in the mercurous nitrate involving Dr. Dixon's name would he brought out by the ammonia indelibly on the other side of the note--he forgot"--Kennedy was now speaking eagerly and loudly--"that the sulpho-cyanide vapors could always be made to bring back to accuse him the words that the ammonia had blotted out."

Before the prosecutor could interfere, Kennedy had picked up the note found in the ammonia-jar beside the dying girl and had jammed the state's evidence into the long-necked flask of sulpho-cyanide vapor.

"Don't fear," he said, trying to pacify the now furious prosecutor, "it will do nothing to the Dixon writing. That is permanent now, even if it is only a tracing."

When he withdrew the note, there was writing on both sides, the black of the original note and something in red on the other side.

We crowded around, and Craig read it with as much interest as any of us:

"Before taking the headache-powder, be sure to place the contents of this paper in a jar with a little warm water."

"Hum," commented Craig, "this was apparently written on the outside wrapper of a paper folded about some sal-ammoniac and quicklime. It goes on:

"'Just drop the whole thing in, _paper and all_. Then if you feel a faintness from the medicine the ammonia will quickly restore you. One spoonful of the headache-powder swallowed quickly is enough.'"

No name was signed to the directions, but they were plainly written, and "_paper and all_" was underscored heavily.

Craig pulled out some letters. "I have here specimens of writing of many persons connected with this case, but I can see at a glance which one corresponds to the writing on this red death-warrant by an almost inhuman fiend. I shall, however, leave that part of it to the handwriting experts to determine at the trial. Thurston, who was the man whom you saw enter the Boncour bungalow as you left--the constant visitor?"

Thurston had not yet regained his self-control, but with trembling forefinger he turned and pointed to Halsey Post.

"Yes, ladies and gentlemen," cried Kennedy as he slapped the telegram that had just come from New York down on the table decisively, "yes, the real client of Kerr & Kimmel, who bent Thurston to his purposes, was Halsey Post, once secret lover of Vera Lytton till threatened by scandal in Danbridge--Halsey Post, graduate in technology, student of sympathetic inks, forger of the Vera Lytton letter and the other notes, and dealer in cyanides in the silver-smithing business, fortune-hunter for the Willard millions with which to recoup the Post & Vance losses, and hence rival of Dr. Dixon for the love of Alma Willard. That is the man who wielded the poisoned pen. Dr. Dixon is innocent."

THE INVISIBLE RAY

BY ARTHUR B. REEVE

"I won't deny that I had some expectations from the old man myself."

Kennedy's client was speaking in a low, full-chested, vibrating voice, with some emotion, so low that I had entered the room without being aware that any one was there until it was too late to retreat.

"As his physician for over twelve years," the man pursued, "I certainly had been led to hope to be remembered in his will. But, Professor Kennedy, I can't put it too strongly when I say that there is no selfish motive in my coming to you about the case. There is something wrong--depend on that."

Craig had glanced up at me and, as I hesitated, I could see in an instant that the speaker was a pract.i.tioner of a type that is rapidly pa.s.sing away, the old-fashioned family doctor.

"Dr. Burnham, I should like to have you know Mr. Jameson," introduced Craig. "You can talk as freely before him as you have to me alone. We always work together."

I shook hands with the visitor.

"The doctor has succeeded in interesting me greatly in a case which has some unique features," Kennedy explained. "It has to do with Stephen Haswell, the eccentric old millionaire of Brooklyn. Have you ever heard of him?"

"Yes, indeed," I replied, recalling an occasional article which had appeared in the newspapers regarding a dusty and dirty old house in that part of the Heights in Brooklyn whence all that is fashionable had not yet taken flight, a house of mystery, yet not more mysterious than its owner in his secretive comings and goings in the affairs of men of a generation beyond his time. Further than the facts that he was reputed to be very wealthy and led, in the heart of a great city, what was as nearly like the life of a hermit as possible, I knew little or nothing, "What has he been doing now?" I asked.

"About a week ago," repeated the doctor, in answer to a nod of encouragement from Kennedy, "I was summoned in the middle of the night to attend Mr. Haswell, who, as I have been telling Professor Kennedy, had been a patient of mine for over twelve years. He had been suddenly stricken with total blindness. Since then he appears to be failing fast, that is, he appeared so the last time I saw him, a few days ago, after I had been superseded by a younger man. It is a curious case and I have thought about it a great deal. But I didn't like to speak to the authorities; there wasn't enough to warrant that, and I should have been laughed out of court for my pains. The more I have thought about it, however, the more I have felt it my duty to say something to somebody, and so, having heard of Professor Kennedy, I decided to consult him. The fact of the matter is, I very much fear that there are circ.u.mstances which will bear sharp looking into, perhaps a scheme to get control of the old man's fortune."

The doctor paused, and Craig inclined his head, as much as to signify his appreciation of the delicate position in which Burnham stood in the case. Before the doctor could proceed further, Kennedy handed me a letter which had been lying before him on the table. It had evidently been torn into small pieces and then carefully pasted together.

The superscription gave a small town in Ohio and a date about a fortnight previous.

Dear Father [it read]: I hope you will pardon me for writing, but I cannot let the occasion of your seventy-fifth birthday pa.s.s without a word of affection and congratulation. I am alive and well--Time has dealt leniently with me in that respect, if not in money matters. I do not say this in the hope of reconciling you to me. I know that is impossible after all these cruel years. But I do wish that I could see you again. Remember, I am your only child and even if you still think I have been a foolish one, please let me come to see you once before it is too late. We are constantly traveling from place to place, but shall be here for a few days.

Your loving daughter,