He fetched from the office a small plate of glass, and a photographic dish in which a piece of thin notepaper was soaking in water.

“This paper,” said Thorndyke, lifting it out and laying it on the glass, “has been soaking all night, and is now quite pulpy.”

He spread a dry sheet of paper over the wet one, and on the former wrote heavily with a hard pencil, “Moakey is a bliter.”  On lifting the upper sheet, the writing was seen to be transferred in a deep grey to the wet paper, and when the latter was held up to the light the inscription stood out clear and transparent as if written with oil.

“When this dries,” said Thorndyke, “the writing will completely disappear, but it will reappear whenever the paper is again wetted.”

The Professor nodded.

“Very ingenious,” said he—­“a sort of artificial palimpsest, in fact.  But I do not understand how that illiterate man could have written in the difficult Moabite script.”

“He did not,” said Thorndyke.  “The ‘cryptogram’ was probably written by one of the leaders of the gang, who, no doubt, supplied copies to the other members to use instead of blank paper for secret communications.  The object of the Moabite writing was evidently to divert attention from the paper itself, in case the communication fell into the wrong hands, and I must say it seems to have answered its purpose very well.”

The Professor started, stung by the sudden recollection of his labours.

“Yes,” he snorted; “but I am a scholar, sir, not a policeman.  Every man to his trade.”

He snatched up his hat, and with a curt “Good-morning,” flung out of the room in dudgeon.

Thorndyke laughed softly.

“Poor Professor!” he murmured.  “Our playful friend Barton has much to answer for.”

VI

THE MANDARIN’S PEARL

Mr. Brodribb stretched out his toes on the kerb before the blazing fire with the air of a man who is by no means insensible to physical comfort.

“You are really an extraordinarily polite fellow, Thorndyke,” said he.

He was an elderly man, rosy-gilled, portly, and convivial, to whom a mass of bushy, white hair, an expansive double chin, and a certain prim sumptuousness of dress imparted an air of old-world distinction.  Indeed, as he dipped an amethystine nose into his wine-glass, and gazed thoughtfully at the glowing end of his cigar, he looked the very type of the well-to-do lawyer of an older generation.

“You are really an extraordinarily polite fellow, Thorndyke,” said Mr. Brodribb.

“I know,” replied Thorndyke.  “But why this reference to an admitted fact?”

“The truth has just dawned on me,” said the solicitor.  “Here am I, dropping in on you, uninvited and unannounced, sitting in your own armchair before your fire, smoking your cigars, drinking your Burgundy—­and deuced good Burgundy, too, let me add—­and you have not dropped a single hint of curiosity as to what has brought me here.”