- " my conscience began to trouble me. I determined to see Vera, tell her all, and find out whether it was she who wanted this statement. I saw her. When at last I told her, she scorned me. I can confirm that, for as I left a man entered. I now knew how grossly I had sinned, in listening to Mose Kimmel. I fled. I disappeared in Maine. I travelled. Every day my money grew less. At last I was overtaken, captured, and brought back here."

He stopped and sank wretchedly down in a chair and covered his face with his hands.

"A likely story," muttered Leland in my ear.

Kennedy was working quickly. Motioning the officers to be seated by Thurston, he uncovered a jar which he had placed on the table. The colour had now appeared in Alma's cheeks, as if hope had again sprung in her heart, and I fancied that Halsey Post saw his claim on her favour declining correspondingly.

"I want you to examine the letters in this case with me," continued Kennedy. "Take the letter which I read from Miss Lytton, which was found following the strange disappearance of the note from Thurston."

He dipped a pen into a little bottle, and wrote on a piece of paper:

What is your opinion about Cross's Headache Cure? Would you recommend it for a nervous headache?

BURGESS THURSTON, c/o Mrs. S. BONCOUR.

Craig held up the writing so that we could all see that he had written what Dixon declared Thurston wrote in the note that had disappeared. Then he dipped another pen into a second bottle, and for some time he scrawled on another sheet of paper. He held it up, but it was still perfectly blank.

"Now," he added, "I am going to give a little demonstration which I expect to be successful only in a measure. Here in the open sunshine by this window I am going to place these two sheets of paper side by side. It will take longer than I care to wait to make my demonstration complete, but I can do enough to convince you."

For a quarter of an hour we sat in silence, wondering what he would do next. At last he beckoned us over to the window. As we approached he said, "On sheet number one I have written with quinoline; on sheet number two I wrote with a solution of nitrate of silver."

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 substituted 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 substituted 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 quantity to decolourise it is invisible when used for writing. But the original colour reappears as the oxygen of the air acts upon the pigment. I haven't a doubt but that my analyses 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 circumstance 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 change, 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 glass 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 vapour of sulpho-cyanide. Here is a long-necked flask of the gas, made by sulphuric acid acting on potassium sulphocyanide. 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 be 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 vapours could always be made to bring back to accuse him the words that the