He signed for the message, tore it open, and read it.  For a moment his face, which had been clouded, smoothed out, and he took a couple of turns up and down the hall, as though undecided.  Finally he crumpled the telegram abstractedly and shoved it into his pocket.  We followed him as he went into the parlor and stood for several moments, looking fixedly on the strangely flushed face of Mrs. Snedden.  “MacLeod,” he said, finally, turning gravely toward us, and, for the present, seeming to ignore the presence of the others, “this amazing series of crimes has brought home to me forcibly the alarming possibilities of applying modern scientific devices to criminal uses.  New modes and processes seem to bring new menaces.”

“Like carbon-monoxide poisoning?” suggested MacLeod.  “Of course it has long been known as a harmful gas, but—­”

“Let us see,” interrupted Kennedy.  “Walter, you were there when I examined Jackson’s car.  There was not a drop of gasolene in the tank, you will recall.  Even the water in the radiator was low.  I lifted the hood.  Some one must have tampered with the carburetor.  It was adjusted so that the amount of air in the mixture was reduced.  More than that, I don’t know whether you noticed it or not, but the spark and gas were set so that, when I did put gasolene in the tank, I had but to turn the engine over and it went.  In other words, that car had been standing there, the engine running, until it simply stopped for want of fuel.”  He paused while we listened intently, then resumed.  “The gas-engine and gas-motor have brought with them another of those unanticipated menaces of which I spoke.  Whenever the explosion of the combustible mixture is incomplete or of moderated intensity a gas of which little is known may be formed in considerable quantities.

“In this case, as in several others that have come to my attention, vapors arising from the combustion must have emitted certain noxious products.  The fumes that caused Ida Snedden’s death were not of carbon monoxide from the stove, MacLeod.  They were splitting-products of gasolene, which are so new to science that they have not yet been named.

“Mrs. Snedden’s death, I may say for the benefit of the coroner, was due to the absorption of some of these unidentified gaseous poisons.  They are as deadly as a knife-thrust through the heart, under certain conditions.  Due to the non-oxidation of some of the elements of gasolene, they escape from the exhaust of every running gas-engine.  In the open air, where only a whiff or two would be inhale now and then, they are not dangerous.  But in a closed room they may kill in an incredibly short time.  In fact, the condition has given rise to an entirely new phenomenon which some one has named ‘petromortis.’”

“Petromortis?” repeated Snedden, who, for the first time, began to show interest in what was going on about him.  “Then it was an accident?”

“I did not say it was an accident,” corrected Craig.  “There is an old adage that murder will out.  And this expression of human experience is only repeated in what we modern scientific detectives are doing.  No man bent on the commission of a crime can so arrange the circumstances of that crime that it will afterward appear, point by point, as an accident.”