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.