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It may be that the police resented the intrusion of an amateur, or that they imagined themselves to be upon some hopeful line of investigation; but it is certain that we heard nothing from them for the next two days. During this time Holmes spent some of his time smoking and dreaming in the cottage; but a greater portion in country walks which he undertook alone, returning after many hours without remark as to where he had been. One experiment served to show me the line of his investigation. He had bought a lamp which was the duplicate of the one which had burned in the room of Mortimer Tregennis on the morning of the tragedy. This he filled with the same oil as that used at the vicarage, and he carefully timed the period which it would take to be exhausted. Another experiment which he made was of a more unpleasant nature, and one which I am not likely ever to forget.

“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.”

They were not long in coming. I had hardly settled in my chair before I was conscious of a thick, musky odour, subtle and nauseous. At the very first whiff of it my brain and my imagination were beyond all control. A thick, black cloud swirled before my eyes, and my mind told me that in this cloud, unseen as yet, but about to spring out upon my appalled senses, lurked all that was vaguely horrible, all that was monstrous and inconceivably wicked in the universe. Vague shapes swirled and swam amid the dark cloud-bank, each a menace and a warning of something coming, the advent of some unspeakable dweller upon the threshold, whose very shadow would blast my soul. A freezing horror took possession of me. I felt that my hair was rising, that my eyes were protruding, that my mouth was opened, and my tongue like leather. The turmoil within my brain was such that something must surely snap. I tried to scream and was vaguely aware of some hoarse croak which was my own voice, but distant and detached from myself. At the same moment, in some effort of escape, I broke through that cloud of despair and had a glimpse of Holmes’s face, white, rigid, and drawn with horror – the very look which I had seen upon the features of the dead. It was that vision which gave me an instant of sanity and of strength. I dashed from my chair, threw my arms round Holmes, and together we lurched through the door, and an instant afterwards had thrown ourselves down upon the grass plot and were lying side by side, conscious only of the glorious sunshine which was bursting its way through the hellish cloud of terror which had girt us in. Slowly it rose from our souls like the mists from a landscape until peace and reason had returned, and we were sitting upon the grass, wiping our clammy foreheads, and looking with apprehension at each other to mark the last traces of that terrific experience which we had undergone.

“Upon my word, Watson!” said Holmes at last with an unsteady voice, “I owe you both my thanks and an apology. It was an unjustifiable experiment even for one’s self, and doubly so for a friend. I am really very sorry.”

“You know,” I answered with some emotion, for I had never seen so much of Holmes’s heart before, “that it is my greatest joy and privilege to help you.”

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