“You will remember, Watson,” he remarked one afternoon, “that there is a single common point of resemblance in the varying reports which have reached us. This concerns the effect of the atmosphere of the room in each case upon those who had first entered it. You will recollect that Mortimer Tregennis, in describing the episode of his last visit to his brother’s house, remarked that the doctor on entering the room fell into a chair? You had forgotten? Well, I can answer for it that it was so. Now, you will remember also that Mrs. Porter, the housekeeper, told us that she herself fainted upon entering the room and had afterwards opened the window. In the second case - that of Mortimer Tregennis himself - you cannot have forgotten the horrible stuffiness of the room when we arrived, though the servant had thrown open the window. That servant, I found upon inquiry, was so ill that she had gone to her bed. You will admit, Watson, that these facts are very suggestive. In each case there is evidence of a poisonous atmosphere. In each case, also, there is combustion going on in the room - in the one case a fire, in the other a lamp. The fire was needed, but the lamp was lit - as a comparison of the oil consumed will show - long after it was broad daylight. Why? Surely because there is some connection between three things - the burning, the stuffy atmosphere, and, finally, the madness or death of those unfortunate people. That is clear, is it not?”

“It would appear so.”

“At least we may accept it as a working hypothesis. We will suppose, then, that something was burned in each case which produced an atmosphere causing strange toxic effects. Very good. In the first instance - that of the Tregennis family - this substance was placed in the fire. Now the window was shut, but the fire would naturally carry fumes to some extent up the chimney. Hence one would expect the effects of the poison to be less than in the second case, where there was less escape for the vapour. The result seems to indicate that it was so, since in the first case only the woman, who had presumably the more sensitive organism, was killed, the others exhibiting that temporary or permanent lunacy which is evidently the first effect of the drug. In the second case the result was complete. The facts, therefore, seem to bear out the theory of a poison which worked by combustion.

“With this train of reasoning in my head I naturally looked about in Mortimer Tregennis’s room to find some remains of this substance. The obvious place to look was the talc shield or smoke-guard of the lamp. There, sure enough, I perceived a number of flaky ashes, and round the edges a fringe of brownish powder, which had not yet been consumed. Half of this I took, as you saw, and I placed it in an envelope.”

“Why half, Holmes?”

“It is not for me, my dear Watson, to stand in the way of the official police force. I leave them all the evidence which I found. The poison still remained upon the talc had they the wit to find it. Now, Watson, we will light our lamp; we will, however, take the precaution to open our window to avoid the premature decease of two deserving members of society, and you will seat yourself near that open window in an armchair unless, like a sensible man, you determine to have nothing to do with the affair. Oh, you will see it out, will you? I thought I knew my Watson. This chair I will place opposite yours, so that we may be the same distance from the poison and face to face. The door we will leave ajar. Each is now in a position to watch the other and to bring the experiment to an end should the symptoms seem alarming. Is that all clear? Well, then, I take our powder - or what remains of it - from the envelope, and I lay it above the burning lamp. So! Now, Watson, let us sit down and await developments.”