As a thief in the night

by R. Austin Freeman

“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.