"Yes--just Cleek of Scotland Yard. And perhaps it was as well that I came when I did, for Captain Kenneth might not have awakened from the next sleep he took, and---- What's that, Colonel? An explanation? Oh, certainly. That's very simple. "In the first place, I discovered that your laboratory was, as you had said, absolutely proof against all outside observation. Clearly, also, there was only one means of entry and that by the simple method of the door. Here I must admit I was puzzled for a while, until I heard through your son of the nitrous oxide and found a strand of blonde hair caught in that Bunsen burner. That explained everything--your headache, Captain, and the second visit of which you knew nothing. Your tooth-pulling operation gave my lady her chance. Probably she had been provided with tubes of that gas, for all her painting tubes smelt of it. Anyhow, I take it that she secured your tube while pretending to faint. After she had succeeded in sending you to sleep, she dressed herself in your clothes, and went downstairs--it was easy enough in the dim light." "What's that? She spoke to Marshall, you say? Oh, yes, I remember that quite clearly. But you must remember that I recognized Elsa Schmidt from the first, and knew her to be a male impersonator of no mean order. She used to be a shining star among the Apaches of Montmartre--but that's another story. Anyhow, having secured the formula, she burnt it and----" "Burnt it?" exclaimed Captain Digby. "Yes, burnt it. The ashes were beside the Bunsen burner as you will see for yourself next time you enter the laboratory. Then all she had to do was to come back and send a picture postcard to her brother Johann, one of the cleverest spies in Europe. By the way, Colonel, he is no more a cripple than I am!" Everyone in the room by this time was looking at Cleek in utter amazement. "Picture postcards you said, Mr. Cleek?" broke in the Colonel, suddenly. "Not those silly little painted things with the fancy borders?" "The very same. And each time they passed through your hands for the postbag, your son's formula passed, too. But that was not your fault. It was simply a matter of that conventional border she was so fond of painting. Look at this one." He drew one from his pocket. "Evidently in this formula you used a combination of that mobile and highly inflammable liquid known as benzene chemically expressed as C6 H6. Now give a glance at this postcard. You will see that it is bordered with multiples of that Benzene Ring, and the dot and dash message underneath gives the exact proportions. All that the lady had to do was to paint a different border round her picture of that shrine and the thing was done.