"Addison," he said hoarsely--"don't joke about it!--don't joke. It was not a grey vapour that strangled Seager...." "Certainly it was not. Seager was strangled by some wholly inoffensive person--we shall probably never know his identity--who had fallen asleep amongst the bushes on the mound, close beside the house...." "But man alive! I've _seen_ the beastly thing, with my own eyes! You've seen it! Wales saw it! Mrs. Ord saw it!..." "Mrs. Ord saw her husband." "Ah! you're coming round to my belief about the Ords!" "Decidedly I am." "But what did Wales see--eh? And what did _I_ see!" "You saw the vapour in operation." The Major fell back in his chair with an expression upon his face which I cannot hope to describe. Words failed him altogether. "I had come prepared for something of the sort," I continued rapidly; "for I have investigated several cases of haunting--notably in the Peak district--which have proved to be due to an emanation from the soil--a vapour. But the effect of such vapour, in the other cases, was to induce delusions of sight, in nearly every instance (although, in two, the delusions were of hearing). "In other words, the person affected by this vapour was drugged, and, during the drugged state, perceived certain visions. I made the mistake, at first, of supposing that Low Fennel came within the same category. The classical analogy, of course, is that of the Sibyls, who delivered the oracular responses from the tripod, under the afflatus of a vapour said to arise from the sacred subterranean stream called Kassotis. The theory is, therefore, by no means a new one!" Major Dale stared dully, but made no attempt to interrupt me. "There are probably many spots, in England alone," I continued, "thus affected; but, fortunately, few of them have been chosen as building-sites. Barrows and tumuli of the stone and bronze age, and also Roman shrines, seem frequently to be productive of such emanations. The barrow beside Low Fennel (and now under the new wing) is a case in point. "Sudden atmospheric changes seem to be favourable to the formation of the vapour. The barrow in Peel Castle, Isle of Man, is peculiarly susceptible to thunder-storms, for instance, whilst that at Low Fennel emits a vapour only after a spell of intense heat, and at the exact moment when the temperature begins to fall again. In the case of a sustained heat-wave, this would take place at some time during each night. "And now for the particular in which the vapour at Low Fennel differs from other, similar emanations. It is not productive of delusions of sight; it induces a definite and unvarying form of transient insanity!" Major Dale moved slightly, but still did not speak.