“Well, let’s pass the motive,” said Bredon. “I’m interested to hear your account of the method.”
“Our mistake from the first has been that of not accepting the facts. We have tried to fit the facts into our scheme, instead of letting the facts themselves guide us. From the first we were faced with what seemed to be a hopeless contradiction. The locked door seemed to make it certain that Mottram was alone when he died. The fact that the gas was turned off seemed to make it clear that Mottram was not alone when he died. There was ground for suspecting either suicide or murder; the difficulty was to make the whole complex of facts fit into either view. We had made a mistake, I repeat, in not taking the facts for our guide. The door was locked; that is a fact. Therefore Mottram was alone from the time he went to bed until the time when the door was broken in. And at the time when the door was broken in the gas was found turned off. Somebody must have turned it off, and in order to do so he must have been in the room. There was only one person in the room—Mottram. Therefore it was Mottram who turned the gas off.”
“You mean in his last dying moments?”
“No, such a theory would be fantastic. Mottram clearly turned the gas off in the ordinary way. Therefore, now, mark this, it was not the gas in Mottram’s room which poisoned Mottram.”
“But hang it all, if it wasn’t in his room”——
“When I say that, I mean it was not the gas which turned on and off in Mottram’s room. For that gas was turned off. Therefore it must have been some independent supply of gas which poisoned him.”
“Such as?”
“Doesn’t the solution occur to you yet? The room, remember, is very low, and the window rather high up in the wall. What is to prevent a supply of gas being introduced from outside and from above?”
“Good Lord! You don’t mean you think that Brinkman”——
“Brinkman had the room immediately above. Since his hurried departure, I have had opportunities of taking a better look round it. I was making some experiments there early this morning. In the first place, I find that it is possible for a man leaning out of the window in Brinkman’s room to control with a stick the position of the window in Mottram’s room—provided always that the window is swinging loose. He can ensure at will that Mottram’s window stands almost shut, or almost fully open.”