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“We now,” Thorndyke resumed, “enter the final stage of the inquiry. Hitherto we have dealt with purely scientific evidence which would have had to be communicated to the jury and which they would have had to take on trust with no convincing help from their own eyes. We had evidence, conclusive to ourselves, that Monkhouse had been murdered by means of a poisoned candle. But we could not produce the candle or any part of it. We had nothing visible or tangible to show to the jury to give them the feeling of confidence and firm conviction which they rightly demand when they have to decide an issue involving the life or death of the accused. It was this something that could be seen and handled that I sought, and sought in vain until that momentous evening when I called at your chambers to return your diary.

“I remember that as I entered the room and cast my eyes over the things that were spread out on the table, I received quite a shock. For the first glance showed me that, amongst those things were two objects that exactly fulfilled the conditions of the final test. There was the wax mould—a part, and the greater part, of one of the suspected candles; and there was the tress of hair—a portion of the body of the person suspected to have been poisoned. With these two objects it was possible to determine with absolute certainty whether that person had or had not been poisoned with arsenic, and if she had, whether the candle had or had not been the medium by which the poison was administered.”

“But,” I said, “you knew from the diary of the existence of the wax mould.”

“I knew that it had existed. But I naturally supposed that the cast had been taken and the mould destroyed years ago, though I had intended to ask you about it. However, here it was, miraculously preserved, against all probabilities, still awaiting completion. Of course, I recognized it instantly, and began to cast about in my mind for some means of making the necessary examination without disclosing my suspicions. For you will realize that I was unwilling to say anything to you about Stella’s death until the question was settled one way or the other. If the examination had shown no arsenic either in the candle or in the hair, it would not have been necessary to say anything to you at all.

“But while I was debating the matter, the problem solved itself. As soon as I came to look at Stella’s unfinished works, I saw that they cried aloud to be completed and that Polton was the proper person to carry out the work. I made the suggestion, which I should have made in any case, and when you adopted it, I decided to say nothing but to apply the tests when the opportunity offered.”

“I am glad,” said I, “to hear you say that you would have made the suggestion in any case. It looked at first like a rather cold-blooded pretext to get possession of the things. But you were speaking of the hair. Can you depend on finding recognizable traces of arsenic in the hair of a person who has been poisoned?”

“Certainly, you can,” he replied. “The position is this: When arsenic is taken it becomes diffused throughout the whole body, including the blood, the bones and the skin. But as soon as a dose of arsenic is taken, the poison begins to be eliminated from the body, and, if no further dose is taken, the whole of the poison is thrown off in a comparatively short time until none remains in the tissues—with one exception. That exception is the epidermis, or outer skin, with its appendages—the finger and toe nails and the hair. These structures differ from all others in that, instead of growing interstitially and being alive throughout, they grow at a certain growing-point and then become practically dead structures. Thus a hair grows at the growing-point where the bulb joins the true skin. Each day a new piece of hair is produced at the living root, but when once it has come into being it grows no more, but is simply pushed up from below by the next portion. Thenceforward it undergoes no change, excepting that it gradually moves upwards as new portions are added at the root. It is virtually a dead, unchanging structure.

“Now suppose a person to take a considerable dose of arsenic. That arsenic becomes diffused throughout all the living tissues and is for a time deposited in them. The growing point of the hair is a living tissue and of course the arsenic becomes deposited in it. Then the process of elimination begins and the arsenic is gradually removed from the living tissues. But in twenty-four hours, what was the growing-point of the hair has been pushed up about the fiftieth of an inch and is no longer a growing structure. It is losing its vitality. And as it ceases to be a living tissue it ceases to be affected by the process of elimination. Hence the arsenic which was deposited in it when it was a living tissue is never removed. It remains as a permanent constituent of that part of the hair, slowly moving up as the hair grows from below, until at last it is snipped off by the barber; or, if the owner is a long-haired woman, it continues to creep along until the hair is full-grown and drops out.”

“Then the arsenic remains always in the same spot?”

“Yes. It is a local deposit at a particular point in the hair. And this, Mayfield, is a most important fact, as you will see presently. For observe what follows. Hair grows at a uniform rate—roughly, a fiftieth of an inch in twenty-four hours. It is consequently possible, by measurement, to fix nearly exactly, the age of any given point on a hair. Thus if we have a complete hair and we find at any point in it a deposit of arsenic, by measuring from that point to the root we can fix, within quite narrow limits, the date on which that dose of arsenic was taken.”

“But is it possible to do this?” I asked.

“Not in the case of a single hair,” he replied. “But in the case of a tress, in which all the hairs are of the same age, it is perfectly possible. You will see the important bearing of this presently.

“To return now to my investigation. I had the bulk of a candle and a tress of Stella’s hair. The questions to be settled were, 1. Was there arsenic in the candle? and 2. Had Stella been poisoned with arsenic? I began by trimming the wax mould in readiness for casting and then I made an analysis of the trimmings. The result was the discovery of considerable quantities of arsenic in the wax.

“That answered the first question. Next, as the tress of hair was larger than was required for your purpose, I ventured to sacrifice a portion of it for a preliminary test. That test also gave a positive result. The quantity of arsenic was, of course, very minute, but still it was measurable by the delicate methods that are possible in dealing with arsenic; and the amount that I found pointed either to one large dose or to repeated smaller ones.

“The two questions were now answered definitely. It was certain—and the certainty could be demonstrated to a jury—that Stella had been poisoned by arsenic, and that the arsenic had been administered by means of poisoned candles. The complete proof in this case lent added weight to the less complete proof in the case of Monkhouse; and the two cases served to corroborate one another in pointing to Barbara as the poisoner. For she was the common factor in the two cases. The other persons—Wallingford, Madeline and the others—who appeared in the Monkhouse case, made no appearance in the case of Stella; and the persons who were associated with Stella were not associated with Monkhouse. But Barbara was associated with both. And her absence from home was no answer to the charge if death was caused by the candles which she had admittedly supplied.

“But complete as the proof was, I wished, if possible, to make it yet more complete: to associate Barbara still more definitely with the crime. In the case of Monkhouse, it was clear that the poisoning always occurred when she was absent from home. But this was not so clear in the case of Stella. Your diary showed that Stella’s relapses coincided pretty regularly with Barbara’s absences; but it was not certain (though obviously probable) that the relapses coincided with the periods of poisoning. If it could be proved that they did coincide, that proof would furnish corroboration of the greatest possible weight. It would show that the two cases were parallel in all respects.

“But could it be proved? If the tress of Stella’s hair had been at my disposal, I had no doubt that I could have decided the question. But the tress was yours, and it had to be preserved. Whatever was to be done must be done without destroying or injuring the hair, and I set myself the task of finding some practicable method. Eventually, I decided, without much hope of success, to try the X-rays. As arsenic is a fairly dense metal and the quantity of it in the deposits quite considerable, it seemed to me possible that it might increase the density of the hairs at those points sufficiently to affect the X-ray shadow. At any rate, I decided to give the method a trial.

“Accordingly, Polton and I set to work at it. First, in order to get the densest shadow possible, we made the tress up into a close cylinder, carefully arranging it so that all the cut ends were in exactly the same plane. Then we made a number of graduated exposures on ‘process’ plates, developing and intensifying with the object of getting the greatest possible degree of contrast. The result was unexpectedly successful. In the best negative, the shape of the tress was faintly visible and was soon to be crossed by a number of perfectly distinct pale bands. Those bands were the shadows of the deposits of arsenic. There could be no doubt on the subject. For, apart from the fact that there was nothing else that they could be, their appearance agreed exactly with what one would have expected. Each band presented a sharp, distinct edge towards the tips of the hairs and faded away imperceptibly towards the roots. The sharp edge corresponded to the sudden appearance of arsenic in the blood when the poisoning began. The gradual fading away corresponded to the period of elimination when the poisoning had ceased and the quantity of arsenic in the blood was becoming less and less from day to day.

“Now, since hair grows at a known, uniform rate, it was possible to convert the distances between these arsenical bands into periods of time; not with perfect exactness, because the rate of growth varies slightly in different persons, but with sufficient exactness for our present purpose. As soon as I looked at those bands, I saw that they told the whole story. But let us follow the method of proof.

“Assuming the rate of growth to be one fiftieth of an inch in twenty-four hours—which was probably correct for a person of Stella’s age—I measured off on the photograph seven inches and a quarter from the cut ends as representing the last year of her life. Of course, I did not know how close to the head the hair had been cut, but, judging by the bands, I assumed that it had been cut quite close to the skin—within a quarter of an inch.”

“I happen to know that you were quite right,” said I, “but I can’t imagine how you arrived at your conclusion.”

“It was quite a simple inference,” he replied, “as you will see, presently. But to return to the photograph. Of the measured space of seven inches and a quarter I took a tracing on sheet celluloid, marking the sharp edges of the bands, the points at which the fading began and the points at which the band ceased to be visible. This tracing I transferred to paper ruled in tenths of an inch—a tenth of an inch representing five days—and I joined the points where the fading began and ended by a sloping line. I now had a diagram, or chart, which showed, with something approaching to accuracy, the duration of each administration of arsenic and the time which elapsed between the successive poisonings. This is the chart. The sloping lines show the fading of the bands.”

He handed me a paper which he had just taken from a drawer and I looked at it curiously but with no great interest. As I returned it after a brief inspection I remarked:

“It is quite clear and intelligible, but I don’t quite see why you took the considerable trouble of making it. Does it show anything that could not be stated in a few words?”

“Not by itself,” he replied. “But you remember that I mentioned having made two other charts, one showing the fluctuations in Stella’s illness and the other showing Barbara’s absences from home during the same period. Here are those other two charts; and now, if you put the three together, your eye can take in at a glance a fact of fundamental importance; which is that the relapses, the absences and the poisonings all coincided in time. The periodicity is strikingly irregular; but it is identical in all three charts. I made these to hand to the jury, and I think they would have been quite convincing, since any juryman could check them by the dates given in evidence, and by inspection of the radiograph of the hair.”

Chart A shows the fluctuations in the illness of Stella Keene during the year preceding her death in October. Divided into intervals of five days.

Chart B shows the distribution of the arsenical bands in Stella Keene’s hair. The steep sides of the curves, towards the tips of the hairs, show the sudden appearance of the deposit, and the sloping sides, towards the roots of the hairs, represent its more gradual fading. Each of the narrow divisions represents five days’ growth.

Chart C shows the periods during which Barbara was absent from home, each absence being represented by a black column. Divided into intervals of five days.

I gazed at the three charts and was profoundly impressed by the convincing way in which they demonstrated the connection between Barbara’s movements and the results of her diabolical activities. But what impressed me still more was the amazing ingenuity with which Thorndyke had contrived to build up a case of the most deadly precision and completeness out of what seemed, even to my trained intelligence, no more than a few chance facts, apparently quite trivial and irrelevant.

“It seems,” I said, “that, so far as you were concerned, the exhumation was really unnecessary.”

“Quite,” he replied. “It proved nothing that was not already certain. Still, the Commissioner was quite right. For the purposes of a trial, evidence obtained from the actual body of the victim is of immeasurably more weight than indirect scientific evidence, no matter how complete. An ordinary juryman might have difficulty in realizing that the hair is part of the body and that proof of arsenical deposit in the hair is proof of arsenic in the body. But the mistake that he made, as events turned out, was in refusing to make the arrest until my statements had been confirmed by the autopsy and the analysis. That delay allowed the criminal to escape. Not that I complain. To me, personally, her suicide came as a blessed release from an almost intolerable position. But if I had been in his place, I would have taken no chances. She would have gone to trial and to the gallows.”

“Yes,” I admitted; “that was what justice demanded. But I cannot be thankful enough for the delay that let her escape. Fiend as she was, it would have been a frightful thing to have had to give the evidence that would have hanged her.”

“It would,” he agreed; “and the thought of it was a nightmare to me. However, we have escaped that; and after all, justice has been done.”

We were silent for a few minutes, during which Thorndyke smoked his pipe with a certain air of attention as if he expected me to put some further questions. And, in fact, there were one or two questions that I wanted to have answered. I began with the simplest.

“I am still a little puzzled by some of the circumstances in this case. The infernal machine I happen to know to have been sent by Barbara, though I don’t understand why she sent it. But Wallingford’s proceedings are a complete mystery to me. What do you suppose induced him to keep a watch on you in that extraordinary fashion? And who was the man who shadowed him? There certainly was such a man, for I saw him, myself. And the same man had been shadowing Miss Norris. What do you make of it all?”

“One can only reason from past experiences,” he replied. “It seems to be a rule that a person who has committed a crime cannot remain quiet and let things take their course. There appears to be an irresistible impulse to lay down false clues and create misleading appearances. It is always a mistake, unless the false clues are laid down in advance, and even then it is apt to fail and unexpectedly furnish a real clue.

“Now Barbara, with all her astonishing cleverness, made that mistake. She laid down a false clue in advance by her absences from home, and the trick certainly worked successfully at the inquest. But it was precisely those absences that put me on the track of the candle, which otherwise might have passed unsuspected. The faked medicine was another false clue which attracted my attention and added to my suspicion concerning the candle. Then, after the event came these other endeavours to mislead. They did neither harm nor good, as it happened, since I had already marked her down as the principal suspect. But if I had been in doubt, I should have followed up those clues and found her at the end of them.

“As to Wallingford, I imagine that she led him to believe that I was employed by you to fix the crime on him and that he was advised to watch me and be ready to anticipate any move on my part; her actual object being to cause him to behave in such a manner as to attract suspicious attention. The function of the private detective—for that is what he must have been—would be to keep Wallingford’s nerves—and Miss Norris’s, too—in such a state that they would appear anxious and terrified and tend to attract attention. The infernal machine was primarily intended, I think, to cast suspicion on one or both of them. That was what I inferred from the total absence of finger-prints and the flagrantly identifiable character of the pistol and the wool.

“But the greatest, the most fatal mistake that Barbara made was the one that is absolutely characteristic of the criminal. She repeated the procedure of a previous crime that had been successful. It was that repetition that was her undoing. Either crime, separately, might have been difficult to fix on her. As it was, each crime was proof of the other.”

Once more we fell silent; and still Thorndyke had the air of expecting some further question from me. I looked at him nervously; for there was something that I wanted to ask and yet I hardly dared to put it into words. For, as I had looked at those charts, a horrid suspicion had taken hold of me. I feared to have it confirmed, and yet I could not let it rest. At last, I summoned courage enough to put the question.

“Thorndyke,” I said, “I want you to tell me something. I expect you know what it is.”

He looked up and nodded gravely.

“You mean about Stella?” said he.

“Yes. How long would she have lived if she had not been poisoned?”

He looked away for a few moments, and, impassive as his face was, I could see that he was deeply moved. At length he replied:

“I was afraid you were going to ask me that. But since you have, I can only answer you honestly. So far as I can judge, but for that accursed ghoul, the poor girl might have been alive and well at this moment.”

I stared at him in amazement. “Do you mean,” I demanded, “that she was not really suffering from consumption at all?”

“That is what it amounts to,” he replied. “There were signs of old tubercular trouble, but there was nothing recent. Evidently she had good powers of resistance, and the disease had not only become stationary, but was practically extinct. The old lesions had undergone complete repair, and there is no reason to suppose that any recurrence would have taken place under ordinary conditions.”

“But,” I exclaimed, hardly able to believe that the disaster had been so overwhelmingly complete, “what about the cough? I know that she always had a more or less troublesome cough.”

“So had Monkhouse,” he replied; “and so would any one have had whose lungs were periodically irritated by inhaling particles of arsenious acid. But the tubercular mischief was quite limited and recovery must have commenced early. And Barbara, watching eagerly the symptoms of the disease which was to rid her of her rival, must have noted with despair the signs of commencing recovery and at last resolved to do for herself what nature was failing to do. Doubtless, the special method of poisoning was devised to imitate the symptoms of the disease; which it did well enough to deceive those whose minds were prepared by the antecedent illness to receive the suggestion. It was a horribly, fiendishly ingenious crime; calmly, callously devised and carried out to its appalling end with the most hideous efficiency.”

After he had finished speaking, I remained gazing at him dumbly, stupefied, stunned by the realization of the enormity of this frightful thing that had befallen. He, too, seemed quite overcome, for he sat silently, grasping his extinct pipe and looking sternly and fixedly into the fire. At length he spoke, but without removing his gaze from the bright embers.

“I am trying, Mayfield,” he said, gently, “to think of something to say to you. But there is nothing to say. The disaster is too complete, too irretrievable. This terrible woman has, so far, wrecked your life, and I recognize that you will carry the burden of your loss so long as you live. It would be a mere impertinence to utter futile and banal condolences. You know what I, your friend, am feeling and I need say no more of that; and I have too much confidence in your wisdom and courage to think of exhortations.

“But, though you have been robbed of the future that might have been, there is still a future that may be. It remains to you now only to shoulder your fardel and begin your pilgrimage anew; and if the road shall seem at first a dreary one, you need not travel it alone. You have friends; and one of them will think it a privilege to bear you company and try to hearten you by the way.”

He held out his hand and I grasped it silently and with a full heart. And the closer friendship that was inaugurated in that hand-clasp has endured through the passing years, ever more precious and more helpful.