ere was B45. He had enjoyed no leave since he had
left England. Yes, he would go down to Rackham Park, and take Harry
Luttrell with him if he could.

Two days later the Commandant Marnier came to see him at the Ritz Hotel.
They dined together in a corner of the restaurant.

"We have solved the problem of those tubes," said Marnier. "They are
nothing more nor less than time-fuses."

"Time-fuses!" Hillyard repeated. "I don't understand."

"Listen!"

Marnier looked around. There was no one near enough to overhear him, if
he did not raise his voice; and he was careful to speak in a whisper.

"Two things." He ticked them off upon his fingers. "First, hydrofluoric
acid when brought into contact with certain forms of explosive will
create a fire. Second, hydrofluoric acid will bite its way through
glass. The thicker the glass, the longer the time required to set the
acid free. Do you follow?"

"Yes," said Hillyard.

"Good! Make a glass tube of such thickness that it will take
hydrofluoric acid four hours and a half to eat its way through. Then
fill it with acid and seal it up. You have a time-fuse which will act
precisely in four hours and a half."

"If it comes into contact with the necessary explosive," Hillyard added.

"Exactly. Now attend to this! Our workmen in our munition factories work
three hours and a half. Then they go to their luncheon."

"Munition factories!" said Hillyard with a start.

"Yes, my friend. Munition factories. We are short of labour as you know.
Our men are in the firing line. We must get labour from some other
source. And there is only one source."

"The neutrals," Hillyard exclaimed.

"Yes, the neutrals, and especially the neutrals who are near to us, who
can come without difficulty and without much expense. We have a good
many Spanish workmen in our munition factories and three of these
factories have recently been burnt down. We have the proof now, thanks
to you, that those little glass tubes so carefully manufactured in
Berlin to last four hours and a half and no more, set the fires going."

"Proof, you say?" Hillyard asked earnestly. "It is not probability or
moral certainty? It is actual bed-rock proof?"

"Yes. For once our chemists had grasped how these tubes could be used,
we knew what to look for when the workmen were searched on entering the
factory. Two days ago we caught a man. He had one of these little tubes
in his mouth and in the lining of his waistcoat, just a lit