A passage followed which disturbed him: 

     “There are other things too.  My magnolia is still in bud.  I
     dread a blight before the flower opens.”

It was a cry of distress—­nothing less than that—­uttered in some moment of intense depression.  Else it would never have been allowed to escape at all.

Hillyard folded up the letter.  He would be going home in any case.  There were those tubes.  There 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?”