“It doesn’t look very promising at first sight,” I remarked.  “What is the Professor’s theory?”

“His theory—­provisionally, of course—­is that the words form the message, and the groups of letters represent mere filled-up spaces between the words.”

“But surely,” I protested, “that would be a very transparent device.”

Thorndyke laughed.  “There is a childlike simplicity about it,” said he, “that is highly attractive—­but discouraging.  It is much more probable that the words are dummies, and that the letters contain the message.  Or, again, the solution may lie in an entirely different direction.  But listen!  Is that cab coming here?”

It was.  It drew up opposite our chambers, and a few moments later a brisk step ascending the stairs heralded a smart rat-tat at our door.  Flinging open the latter, I found myself confronted by a well-dressed stranger, who, after a quick glance at me, peered inquisitively over my shoulder into the room.

“I am relieved, Dr. Jervis,” said he, “to find you and Dr. Thorndyke at home, as I have come on somewhat urgent professional business.  My name,” he continued, entering in response to my invitation, “is Barton, but you don’t know me, though I know you both by sight.  I have come to ask you if one of you—­or, better still, both—­could come to-night and see my brother.”

“That,” said Thorndyke, “depends on the circumstances and on the whereabouts of your brother.”

“The circumstances,” said Mr. Barton, “are, in my opinion, highly suspicious, and I will place them before you—­of course, in strict confidence.”

Thorndyke nodded and indicated a chair.

“My brother,” continued Mr. Barton, taking the profferred seat, “has recently married for the second time.  His age is fifty-five, and that of his wife twenty-six, and I may say that the marriage has been—­well, by no means a success.  Now, within the last fortnight, my brother has been attacked by a mysterious and extremely painful affection of the stomach, to which his doctor seems unable to give a name.  It has resisted all treatment hitherto.  Day by day the pain and distress increase, and I feel that, unless something decisive is done, the end cannot be far off.”

“Is the pain worse after taking food?” inquired Thorndyke.

“That’s just it!” exclaimed our visitor.  “I see what is in your mind, and it has been in mine, too; so much so that I have tried repeatedly to obtain samples of the food that he is taking.  And this morning I succeeded.”  Here he took from his pocket a wide-mouthed bottle, which, disengaging from its paper wrappings, he laid on the table.  “When I called, he was taking his breakfast of arrowroot, which he complained had a gritty taste, supposed by his wife to be due to the sugar.  Now I had provided myself with this bottle, and, during the absence of his wife, I managed unobserved to convey a portion of the arrowroot that he had left into it, and I should be greatly obliged if you would examine it and tell me if this arrowroot contains anything that it should not.”