"How do you know?"

"Well, I worked out this experiment carefully. I've calculated
just how strong the new gas will be, and--"

"Ah, that's just it. It's a new gas, and you've never yet
succeeded in making it, have you?"

"No; but--"

"And it takes a different combination of chemicals to make it
from any you ever experimented with before, doesn't it?" asked
Mark.

"It does. But--"

"Yes, and I don't see how you can tell, with any amount of
calculation, just how much force will develop from those
chemicals, as no one ever put them together before."

"Well, maybe I can't," admitted Jack. "But this tube is very
strong, and even if it does break nothing very serious can
happen."

"Unless the gas you expect to generate is stronger than you have
any idea of."

"Well, I'm going to do it. I've got half an hour before
Professor Lenton and his class comes in, and that's time enough.
Here, just hold this rubber tube under this jar, will you? And