“That’s all right,” said Hillyard; and Fairbairn gently slid the sheet into the dish in front of Hillyard.  And for a while nothing happened.

“It’s a clever trick, isn’t it?” Hillyard used the words again, but now with a note of nervousness.  “No unlikely paraphernalia needed.  Just a copying pencil and some vinegar, which you can get anywhere.  Yes, it’s a clever trick!”

“If it works,” Fairbairn added bluntly.

Both men watched the dish anxiously.  The paper remained blank.  The solution did not seem to work.  It was the first time they had ever made use of it.  The coast slid by unnoticed.

“Lopez was certain,” said Fairbairn, “quite certain that this was the developing formula.”

Hillyard nodded gloomily, but he did not remove his eyes from that irresponsive sheet.

“There may be some other ingredient, something kept quite secret—­something known only to one man or two.”

He sat down, hooking his chair with his foot nearer to the table.

“We must wait.”

“That’s all there is to be done,” said Fairbairn, and they waited; and they waited.  They had no idea, even if the formula should work, whether the writing would flash up suddenly like an over-exposed photographic plate, or emerge shyly and reluctantly letter by letter, word by word.  Then, without a word spoken, Fairbairn’s finger pointed.  A brown stain showed on the whiteness of the paper—­just a stroke.  It was followed by a curve and another stroke.  Hillyard swiftly turned the oblong developing dish so that the side of it, and not the end, was towards him now.

“The writing is across the sheet,” he said, and then with a cry, “Look!”

A word was coming out clear, writing itself unmistakably in the middle of the line, at the bottom of the sheet—­a signature.  Zimmermann!

“From the General Staff!” said Hillyard, in a whisper of excitement.  “My word!” He looked at Fairbairn with an eager smile of gratitude.  “It’s your doing that we have got this—­yours and Lopez Baeza’s!”

Miraculously the brown strokes and curves and dots and flourishes trooped out of nothing, and fell in like sections and platoons and companies with their due space between them, some quick and trim, some rather slovenly in their aspect, some loitering; but in the end the battalion of words stood to attention, dressed for inspection.  The brown had turned black before Hillyard lifted the letter from the solution and spread it upon a sheet of blotting paper.

“Now let us see!” and they read the letter through.

One thousand pounds in English money were offered for reliable information as to the number of howitzers and tanks upon the British front.

A second sum of a thousand pounds for reliable information as to the manufacture of howitzers and tanks in England.

“So far, it’s not very exciting,” Hillyard remarked with disappointment, as he turned the leaf.  But the letter progressed in interest.