Just as he concluded the test, some one opened the laboratory door. It was Professor Lenton.
"I have a telegram for you," he said.
"A telegram?"
"Yes. It just arrived."
Jack tore open the yellow envelope.
"It's from Professor Henderson," he said.
"Is anything the matter?" asked Mark.
"I don't know," answered Jack. "It says: 'Come home at once.' I wonder what's wrong?"
"I hope nothing serious," said Professor Lenton.
"You may both prepare to leave this afternoon. I am sorry. Let me hear from you when you reach Professor Henderson. I trust nothing has happened to him. He is too great a scientist for us to lose."