He turned over the leaves of the “Trades” section and resumed: “I see there are four rice-mills in London, of which the largest is Carbutt’s at Dockhead. Let us look at the spice-factories.” He again turned over the leaves and read down the list of names. “There are six spice-grinders in London,” said he. “One of them, Thomas Williams & Co., is at Dockhead. None of the others is near any rice-mill. The next question is as to the flour-mill. Let us see. Here are the names of several flour millers, but none of them is near either a rice-mill or a spice-grinder, with one exception: Seth Taylor’s, St. Saviour’s Flour Mills, Dockhead.”
“This is really becoming interesting,” said I.
“It has become interesting,” Thorndyke retorted. “You observe that at Dockhead we find the peculiar combination of factories necessary to produce the composite dust in which this coat has hung; and the directory shows us that this particular combination exists nowhere else in London. Then the graphite, the cocoa and the hops tend to confirm the other suggestions. They all appertain to industries of the locality. The trams which pass Dockhead, also, to my knowledge, pass at no great distance from the black-lead works of Pearce Duff & Co. in Rouel Road, and will probably collect a few particles of black-lead on the seats in certain states of the wind. I see, too, that there is a cocoa factory—Payne’s—in Goat Street, Horsleydown, which lies to the right of the tram line going west, and I have noticed several hop warehouses on the right side of Southwark Street, going west. But these are mere suggestions; the really important data are the rice and flour mills and the spice-grinders, which seem to point unmistakably to Dockhead.”
“Are there any private houses at Dockhead?” I asked.
“We must look up the ‘Street’ list,” he replied. “The Yale latch-key rather suggests a flat, and a flat with a single occupant, and the probable habits of our absent friend offer a similar suggestion.” He ran his eye down the list and presently turned to me with his finger on the page.
“If the facts that we have elicited—the singular series of agreements with the required conditions—are only a string of coincidences, here is another. On the south side of Dockhead, actually next door to the spice-grinders and opposite to Carbutt’s rice-mills, is a block of workmen’s flats, Hanover Buildings. They fulfil the conditions exactly. A coat hung in a room in those flats, with the windows open (as they would probably be at this time of year), would be exposed to the air containing a composite dust of precisely the character of that which we have found. Of course, the same conditions obtain in other dwellings in this part of Dockhead, but the probability is in favour of the buildings. And that is all that we can say. It is no certainty. There may be some radical fallacy in our reasoning. But, on the face of it, the chances are a thousand to one that the door that that key will open is in some part of Dockhead, and most probably in Hanover Buildings. We must leave the verification to Miller.”
“Wouldn’t it be as well to look at the tram-ticket?” I asked.
“Dear me!” he exclaimed. “I had forgotten the ticket. Yes, by all means.” He opened the envelope and, turning its contents out on the bench, picked up the dingy slip of paper. After a glance at it he handed it to me. It was punched for the journey from Tooley Street to Dockhead.
“Another coincidence,” he remarked; “and by yet another, I think I hear Miller knocking at our door.”
It was the superintendent, and, as we let him into the room, the hum of a motor-car entering from Tudor Street announced the arrival of Mrs. Chater. We waited for her at the open door, and, as she entered, she held out her hands impulsively.