d!"

"Well, John," said M'Allister, "you're as welcome to mine as if it were
your own, and it's fine strong stuff too. And you can have some of my
Navy plug as well," he added with a grin; "you'll find it rare good
chewing."

"I simply cannot take the Professor's tobacco," said John; then, angrily
turning upon poor M'Allister, he cried, "And as for your filthy stuff,
it's a downright insult to offer it to me!"

"John! John!" I implored, "do be reasonable; it's not at all like you to
talk in this rude way, and you must know we really cannot go back now!"

"Reasonable!" he sneered. "Do you call it reasonable, Professor, to ask
a man who is a lover of his pipe to go all the way to Mars and stay
there for months without any tobacco!"

"Well, you will not accept mine, although you know perfectly well that
you are heartily welcome to it. It's not your own particular brand, it
is true, but it is a real good one. However, most likely you will find
some on Mars; there's plenty of vegetation on that planet, without a
doubt."

"Vegetation be hanged!" he angrily exclaimed. "What am I to do in the
meantime? As for tobacco growing upon Mars--why, sir, I'd bet my bottom
dollar that, outside our own world, there's no place in the whole
universe where anything equal to my superb mixture can be produced. It's
no use talking, Professor; as I said before, we must go back."

"We cannot go back," I replied sternly, for by this time I was becoming
very irritated at his obstinacy. "The idea of going back so many million
miles merely to fetch tobacco! Remember, we have travelled at least
57,000,000 miles on the way to our destination!"

John strode up and down, becoming more and more excited every minute,
and was soon quite raging; yet it seemed most singular that the more
John raged the more M'Allister laughed. I looked from one to the other
in amazement and the most utter perplexity at this extraordinary change
in their behaviour. Then all at once I saw a gleam of light, so to
speak, and the solution of the mystery became clear to me.

The air we had so long been breathing when in the air-chamber, and when
we made use of our air-bottles, was very similar to what is popularly
known as "laughing-gas"; and undoubtedly we were all more or less
experiencing the cumulative effects of the constant mild doses we had
inhaled. Laughing-gas acts in a different manner upon persons of
different temperaments: some will keep laughing, moderately o