“You come at a crisis, Watson,” said he. “If this paper remains blue, all is well. If it turns red, it means a man’s life.” He dipped it into the test-tube and it flushed at once into a dull, dirty crimson. “Hum! I thought as much!” he cried. “I will be at your service in an instant, Watson. You will find tobacco in the Persian slipper.” He turned to his desk and scribbled off several telegrams, which were handed over to the pageboy. Then he threw himself down into the chair opposite, and drew up his knees until his fingers clasped round his long, thin shins.

“A very commonplace little murder,” said he. “You’ve got something better, I fancy. You are the stormy petrel of crime, Watson. What is it?”

I handed him the letter, which he read with the most concentrated attention.

“It does not tell us very much, does it?” he remarked, as he handed it back to me.

“Hardly anything.”